well in old school, it could take millions of XP to reach some levels, so you might go several sessions without leveling.
If by old school, you mean 1e AD&D, the pace of leveling is entirely a function of what percentage of a character's XP is expected to be treasure. A good way to look at it is to compare the following methods:
a) A wilderness campaign which is built directly from the tools 1e Monster Manual, using the '% in lair', number encountered, and treasure types to set the amount of treasure.
b) A dungeon campaign which is built directly for the tools in the appendix of the 1e Dungeon Master's guide to randomly generate a dungeon, using the random treasures described in those tools to place treasure.
c) An adventure path campaign which is built directly from published modules like the G series.
In the case of the strict treasure type campaign, about 2/3rds of the XP will come from treasure and about 1/3rd from killing monsters. Pace of leveling will be glacially slow by modern standards, and will tend to decrease over time as hauls of treasure large enough to make a big dent in your XP needs get rarer and rarer. By 10th level, pace of leveling will be years of weekly game play per level. Magic items are extremely rare and make you cheer when you find one.
In the case of the random dungeon delving campaign, more than 4/5ths of the XP will come from treasure and less than 1/5th from killing monsters. Pace of leveling will still be fairly slow by modern standards, but not as slow as in the first campaign, nor will the slow down as you gain levels be anywhere as large since finding high level foes and rich treasure hordes is just as matter of efficiently getting down to the lower levels of the dungeon. Getting magic items will still be somewhat hard.
In the case of playing published modules, more than 9/10ths of the XP will come from treasure and less than 1/10th from killing monsters. In effect, the monsters in this campaign are all several times richer than the ones in the treasure type campaign. Pace of leveling will be extremely rapid and comparable to modern adventure paths. Magic items will be super abundant, so abundant that the players may consider selling them to gain additional XP.
Then, at lowest levels, the monsters were worth about 1/150th-175th what you needed to level, so the earliest levels were really slow too.
This sentence is actually the reason I'm responding, because it seems a bizarre misunderstanding of old school. Sure, a goblin might be only 13-20XP, and a party of 6 might need nearly 12000 XP level up. But this in no fashion implies that 1st level parties had to kill 600 goblins to level up. Since a tribe of goblins on average had treasure worth 2 times the XP of the tribe itself, that drops it to 200 goblins for a party to level up. Which means that even lowly goblins supplied about 1/30th of the XP you needed to level in even the strictest by the book campaign. But as I said above, if you played with published modules or dungeon delves, that pace increased tremendously. In a dungeon oriented campaign, goblins would be holding treasure worth 4 times their worth in XP, meaning goblins supplied perhaps 1/15th of the XP you needed to level. By the time you get to something like a published module, those goblins held treasure worth 10 times their own worth, which means each goblin was perhaps 1/9th of the XP you needed to level.
We ran something like 1-2 encounters every 4 hours. At level 1, 1-2 bad guys per party member, is really only like 14-30 xp per person, so in an 8 hour Mt Dew session, you have what? 56-120 xp? How is that going to level you when you need 1200-3000? Like if your bad guys are orcs, and they are worth 15xp a piece, and you have 4 party members, needing 1250, 2000, 1500, and 2500 xp, how many orcs do you have to kill? 484 orcs. That's a pretty effective Lord of the Rings scene- probably several scenes. But if you are level 12, you need 250,000+ a piece right? so that's like 20 dragons per player character PER LEVEL.
Err... treasure.... the single biggest source of XP in 1e no matter how your DM ran the game? (Also, if those orcs have javelins or other ranged weapons, they are worth more XP.) Also, again, orcs on average have treasure worth several times their XP, not counting any XP that you can get for hauling a wagon load of discount/slightly used weapons and armor back to the haven. I assure you, no 1e AD&D party ever had to kill a 100+ orcs each to reach 2nd level. For one thing, they'd never have survived.
As for what a level means, I agree with you that post 1e, the meaning and value of being high level took a nose dive. A 15th level character in 1e AD&D who'd earned it, was a major global mover and shaker. The same character in 2e was a mid-level manager in a world were all the good things were owned by NPCs, and most the important things were done by NPCs. The same character in later editions was a gerbil on the leveling treadmill, still doing the exact same things at higher level that he did at lower level, fighting foes that were just scaled of versions of their lower level foes.
ADDENDUM: Based on your mention of things like Dark Sun and Planescape, it seems likely your idea of "Old School" is my idea of New School, and you were playing 2e AD&D.
In that case, if you are playing strictly by the book, substitute character, session, roleplay, problem solving, and story awards for treasure, with the exception of the thief class which still earns XP from treasure. In this case, how fast you level is entirely a function of how much XP the DM is choosing to award for things other than killing the monsters. XP is largely a matter of fulfilling the DM's story goals, and playing your role. Pace of leveling is story oriented.
