Here's a simple question about OD&D / AD&D / B/XD&D / RC D&D. (I imagine the question would apply to more recent versions of the game too; it isn't really a system specfic topic).
In terms of sheer numbers, it takes far more XP to level up the higher up a character goes. Using Cook's D&D Expert (1981) rulebook, consider a simple example: the Fighter. He needs 2000 XP to get to 2nd level, but 120,000 XP (!!) to go from level 8 to 9. Just looking at the numbers, it would seem rather obvious that it takes him WAAAAAAAAAAAAAAY longer to go from 8 to 9 than it did to go from 1 to 2.
This really depends on the DM and how it runs his game. Prior to 3e, in most campaigns, far and away the largest percentage of XP that a player would earn would come from treasure. In campaigns however where a fairly large percentage of XP came from monster killing (say about half) leveling up would be very slow. So the rate at which a player leveled up depended in large part on to what extent the DM made treasure available.
At one extreme, the adventure paths of the day - Desert of Desolation, Against the Giants, Temple of Elemental Evil, etc. - had been designed so that sufficient treasure was available to ensure that the party would level up in time to face the next episode of the adventure path. A DM running players through these modules or emulating them - as was a very common practice - would very likely not see any change in the rate of advancement over time. In this way, the game would run as 3.X was designed to run by default.
But on the opposite extreme, a DM adhering to the random treasure generation tables in the back of the monster manual, and which was careful to take the advice in the pages of the DMG both in terms of not making treasure too abundant, discounting the XP for treasure obtained when the challenges were weak relative to player level, and of rarely placing treasure which was ungaurded and unhidden, would likely see a drop off in the rate that PC's leveled up over time starting around 8th level and then becoming even slower at 10th level both because of the increasing amount of XP required and because of the diminishing returns of an average encounter relative to the players. Just because you leveled up didn't make wealthy monsters automatically less rare, though of course lower levels of the dungeons and further reaches of the wilds were more likely to contain powerful (and hense wealthy) monsters. This same DM also probably saw a much slower rate of advancement across all of play, and if the DM was really strict and adhered to a very low level of play its quite concievable in 1e AD&D play not not expect anyone to ever level up (see math below). While I didn't ever play in a group where getting to second level was a challenge in itself, I can recall playing in campaigns where as you reached name level, you gained a new level roughly every 25-30 sessions. It was a big deal to level up once you were high level.
Somewhere in the middle would have been using the treasure generation and placement suggested by the random dungeon generator in the appendix's of the DMG. I typically played 'in the middle' as it were, and I never played in a campaign with truly strict exceedingly slow leveling at all levels even though the text could be construed at times to encourage it.
So, lets look at the math. Obviously, if the goal - like in an adventure path - is to ensure leveling up, that's easily enough done - just plop down the treasure and goodie bags. But the 'natural rate' suggested by the rules was very different.
A typical low level foe, like a goblin was worth on average 9 XP, or 11 XP if it had a missile weapon. It's share of the tribes treasure combined with its personal wealth worked out to on average: 6 cp, 16 sp, 1 ep, 0.4% chance of 1 gem, 0.2% chance of 1 jewelry, and 0.09% chance of 1 magic item.
The coins themselves are worth only a little more than a g.p., or 1 XP, but the jump between the value of coins and gems, jewelry, and magic items is enormous in 1e. To the extent that after fighting a tribe of goblins, most of the XP will likely come from a lucky find of those things (if they are present). One nice peice of jewelry will level up a 1st level party, for example. If the DM is using truly random placement, a single peice of jewelry is worth on average 3544.5 g.p. Thus, that mere 0.2% chance of a peice of jewelry translates to ~7 XP by itself - nearly doubling the expected XP for fighting a goblin. Working out the exact value of an average gem is a fairly complex problem I'd probably write a program for, but from experience I can say that its about 300 g.p. So the 0.4% chance of a gem works out to about 1 additional expected XP. Together with the vanishingly small chance of a magic item, we're looking at about 18-20 XP per goblin slain - about half in treasure and half in XP. For a party of six PC's needing say 7200 XP to get the thief to level 2, we're looking at slaying an entire tribe of 600 or so goblins just to get the first member of the party to level 2. Obviously, goblins aren't going to cut it as a leveling device.
A bugbear, for example, is worth about 120 XP (or 145 XP with a bow). The expected treasure from killing a bugbear is 120 cp, 52 sp, 37 ep, 29 gp, 6% chance of 1 gem, 2.4% chance of 1 jewelry, 0.5% chance of 1 magic armor or weapon. Not counting any mundane equipment like weapons that could be salvaged and brought back to town, that's about 149 XP for the treasure. So, while a 1st level party could hardly expect to face a large number of bugbears at a time, it would only take about 26 Bugbears to get enough XP to level up the 1st level thief.
The thing to note here is that in typical 1e play, Bugbears might be a foe you'd face as a boss monster type fight at 1st level, and still expect to be encountering (ordinary) bugbears as foes at 10th level when your game had gone 'epic'. You wouldn't necessarily be facing a diet of solely wealthy monsters like dragons, so typically your leveling would go by fits and starts as you found particularly rich troves of treasure. In fact, my numbers here are really obscuring just how fitful leveling truly was, because the random treasure generation tables typically resulted in feast or famine situations where one monster had nothing, and another monster has by itself enough XP via treasure to level up a low level party.
Just a few more examples:
Ghoul, worth about 83 XP for killing it; typical treasure: 173 cp, 67 sp, 48 ep, 38 gp, 10% chance of 1 gem, 4% chance of 1 jewelry, 0.7% chance of 1 magic weapon, 9.6% chance of 1 scroll.
Ettin, worth about 2720 XP for killing it; typical treasure: 1145 cp, 820 sp, 100 ep, 1960 gp, 35% chance of 1 gem, 16% chance of 1 jewelry, 8% chance of 1 magic item
Cloud Giant, worth about 5194 XP for killing it; typical treasure: 78 cp, 464 sp, 250 ep, 3821 gp, 143 pp, ~1 gem, 7.5% chance of 1 jewelry, 21% chance of 1 magic item, 7% chance of 1 scroll
Orcus, 63900 XP; typical treasure: 1050 sp, 375 ep, 2 potions, 40 gems, 14 pieces of jewelry, 2 potions, ~1 scroll, 70% chance of 1 magic item (not potions or scrolls) and possibly the Wand of Orcus.