I've done that before, many times, it's usualy on the forth day that I realise, and usualy because i sudenly get bored and decide to eat something, and suddenly remember when I last ate. The most I have done is 5 days, but I was so bored and coulden't sleep, it was half 4 in the morning, so it could have been almot 6 but boredom and no sleep won over.
Thinking about it, if I had a free week and a brand new, very long game, I think I probably could go a week without eating. I might give it a go when Skyrim comes out, I'm already deeply unhealthy, so I don't think this will affect my health too much.