The experience required for each level is;
E = 1000*(level^3 - level)/3
see
Wisdom - is it worth the skill points?The experience gained from monsters is proportional to the difference in levels (if you are much higher, you get less experience) and monsters do not respawn, so there is an effective maximum level of about 55, assuming you do not cheat or run into a bug with
Experience Bombs. Most people finish the game with a character level in the mid forties to low fifties.
You can Resurrect a creature, run/teleport away so it turns hostile, return and kill it again to get more experience, but this could get boring fast.