What is the typical range of the highest ability scores you will see in a game excluding magical items (not tomes)? 30? 40?
The highest you can see at level 20th, assuming an LA +0 player race that put all commonplace resources into it, is 30:
- 18 maximum possible base
- +2 racial
- +5 from levels
- +5 inherent (tomes or wish)
To this you would typically add a +6 enhancement bonus from an item, for a total of 36.
Please note that 3.5 responds extremely poorly to these and other crucial magic items being removed. 3.5 is not equipped to handle an item-less game, and the amount of work necessary to convert it into a system that can is far greater than simply learning a new system designed for that kind of story. All of the numbers, as well as the particular challenges and obstacles, are designed assuming you have quite a lot of magical items.
Anyway, that said, there are ways to go higher; some races get +4 to an ability score, or some templates allow you to get there (a dragonborn mongrelfolk gets a staggering +6 to Constitution), and of course high LA or RHD creatures often have larger bonuses. And some classes get into things; a barbarian has up to +8 to Strength and Constitution from Mighty Rage, a war hulk gets a monstrous +20 over the course of 10 levels (but zero BAB for those same levels, so that evens out somewhat).
And, of course, further shenanigans are always possible. It’s possible to literally pump ability scores as high as you want if you abuse enough material, and it’s even possible to do that right from level 1.
“Drawing a line” is rarely necessary, so long as the above ludicrous shenanigans are avoided (they’ll be obvious when employed, since they typically involve a feedback loop that gets endlessly-increasing bonuses). Ability score bonuses tend to be expensive, for good reason, so in most cases if someone has them, they’ve earned them. I certainly would not deny a dragonborn mongrelfolk barbarian his 42 Constitution; that’s clearly a character focused on having the highest Constitution available.