Man in the Funny Hat
Hero
As noted above, 2E charts generally covered only up to lvl 20 (or perhaps had a listing of "20+"), but there was no rule that limited advancement to 20 as a hard cap. People may have THOUGHT there was supposed to be a cap because the charts all topped out at 20, but this was not actually the case.
As far as when campaigns ended or at what levels PC's topped out prior to 2E it depends entirely on the campaign. Personally, I played in a campaign that lasted at least 10 years but the xp system we used made for slow advancement and less than a handful of player characters made it to 20 or higher. However, reading letters to Dragon Magazine it was clear that many people ran campaigns with characters well into the 20's or higher. Extreme cases had characters with hundreds of levels but I really suspect they were just doing it wrong and had nobody in the room who was more experienced in these things to tell them.
I surmise that most campaigns did end with PC's somewhere in the low teens or often before reaching title level. Games ended (and still do) for lots of reasons. They often seem to just sort of fizzle out - my own campaigns often did during the summer months when we were out of school and players preferred to be doing other things on Saturdays rather than spending noon-to-midnight playing D&D with me. But players move away. They lose interest in the game. The DM runs out of enthusiasm or ideas - maybe someone else then steps in as DM for a while but the previous campaign never re-starts. Schedules change. Players get jobs, take trips, get girlfriends, etc. Occasionally people decide not to join in the move to a new edition. They get addicted to WoW or other games (computer or whatever).
If anything the odds are actually AGAINST any given campaign running for more than, say, 6-9 months. How many levels PC's accumulate in that time is dependent upon how often people play, how long the sessions last, how much they concentrate on combat as opposed to dedicated roleplaying, how fast they get through combats, how much xp the DM awards, and since we're talking about 1E here - how many encounters with level-draining undead without recourse to restoration? How much of a jerk is the DM about inflicting xp penalties for "alignment changes" or other disapproved PC behaviors? How many PC's just simply died early and often, because that sort of thing happened A LOT in AD&D but it was expected and accepted as just part of the game? You could play 1E for YEARS without anyone ever even making title level simply because the character would die or be retired so the player could try a new PC.
As far as when campaigns ended or at what levels PC's topped out prior to 2E it depends entirely on the campaign. Personally, I played in a campaign that lasted at least 10 years but the xp system we used made for slow advancement and less than a handful of player characters made it to 20 or higher. However, reading letters to Dragon Magazine it was clear that many people ran campaigns with characters well into the 20's or higher. Extreme cases had characters with hundreds of levels but I really suspect they were just doing it wrong and had nobody in the room who was more experienced in these things to tell them.

I surmise that most campaigns did end with PC's somewhere in the low teens or often before reaching title level. Games ended (and still do) for lots of reasons. They often seem to just sort of fizzle out - my own campaigns often did during the summer months when we were out of school and players preferred to be doing other things on Saturdays rather than spending noon-to-midnight playing D&D with me. But players move away. They lose interest in the game. The DM runs out of enthusiasm or ideas - maybe someone else then steps in as DM for a while but the previous campaign never re-starts. Schedules change. Players get jobs, take trips, get girlfriends, etc. Occasionally people decide not to join in the move to a new edition. They get addicted to WoW or other games (computer or whatever).
If anything the odds are actually AGAINST any given campaign running for more than, say, 6-9 months. How many levels PC's accumulate in that time is dependent upon how often people play, how long the sessions last, how much they concentrate on combat as opposed to dedicated roleplaying, how fast they get through combats, how much xp the DM awards, and since we're talking about 1E here - how many encounters with level-draining undead without recourse to restoration? How much of a jerk is the DM about inflicting xp penalties for "alignment changes" or other disapproved PC behaviors? How many PC's just simply died early and often, because that sort of thing happened A LOT in AD&D but it was expected and accepted as just part of the game? You could play 1E for YEARS without anyone ever even making title level simply because the character would die or be retired so the player could try a new PC.