julian,
If you're speaking of Live Chess, you can't. However, you can play a REALLY long game (by Internet standards, that is). You can set the time controls to 120 60 (that's 120 minutes per player with 60 seconds of bonus time added back to your clock after each move you make). This is roughly equivalent to G/160 (160 minutes per side, no bonus). That's a considerable amount of time.
However, you'd better have a friend interested in playing such time controls otherwise you may have to wait a while to have your challenge accepted randomly. In my experience, most players here aren't interested in such a long game with a stranger.
Best of luck.
How do I get into a no time limit game?