For nearly any 1 player game, 99 levels is more than enough. Even if the player maxes their level, it's usually near the end if you did the exp right, and they can enjoy "wtfomgpwning" the final enemies.
-edit
Although if you absolutely have to keep the exp relatively high, and want to extend the levels a bit more, you could add an exp "buying" system, where you can spend exp to raise stats. A simple window with some buttons would do the trick. This way a player can raise their stats using exp, increasing the time it takes to max their level, and allowing them to increase in power just as they would if they leveled to say, 300.
I haven't messed with it, but if possible, you could edit where exp is given to the character, and cut it in half, and have the other half put into a "spending" variable. So not only does the player get 99 levels, they also get 99 levels worth of "buying".