That is a question that begs a lot of other questions, and can't really be sufficiently answered without having first decided on a few other design decisions. How are you gaining XP? What actions cause you to gain XP? Is it gained, or is it taken from some other pool? Is the amount of experience finite or infinite? Do NPCs gain and lose experience as well? How difficult do you want it to be for the character to advance a level? What are the benefits of gaining a level of experience?

That said, a lot of MUDs just take a number to start with that seems reasonable and do it a curve off that. Some of them are steeper than others. It all has to be balanced against how easy or difficult experience is to get, how much of it is out there, and what you get for gaining enough experience.

@Quix - this is exactly how my game is right now, with the caveat that I have skills that let a player refine those coarse readings to numbers.

Maya/Rudha

If you want a fun and amusing system to play around with, and see if your players love you or hate you, try this.

player.level += (random( MAX_LEVEL * 100 ) > ( player.level * 100 )) ? 1 : 0;

So, whenever a player kills anything (or does anything that would grant exp), don't bother tracking the exp.. just run that.

It's all about perception, right? :)

EDIT: typo, > works better. *grin*

Here's something that I came across (it's apparently the Borderlands XP formula):

xp = int(60 * ((level ** 2.6) - 1)) + 1

I like it.

EDIT: Here's a chart.

Bigger version.

Nice!

chance = min(0,target.level - level)
advance if random(ML) < chance

Max, perhaps? Otherwise, if the target's level is greater than yours, your chance is always zero, and if your level is greater, your chance is negative.

Yeah. Max.