From the review:
The main dungeon that comes with Torchlight consists of 35 levels, and you get a new theme after every five levels (so the undead levels look different than the goblin levels, for example). The enemies in the main dungeon are fixed, with Ordrak always being level 30, but there are optional levels you can visit, including (I think) an infinitely large shadow vault that opens up after you've defeated Ordrak once. That means you can keep playing your character for about as long as you'd like, and it also means you can adjust the game's difficulty on the fly. If the monsters in the main dungeon are too tough for you, then you can hit some optional levels for a while and build up your strength.