you know, I see some really good responses here. Personally I don't put a number on it. Instead I think of my
bankroll (Live and online) as a high volatility investment account that I control with every action I make. from what buy-in to sit down at, to what tournaments to play in. all the way down to making a pre-flop raise to making a hero call or fold on the river.
Now, with that said, all investment accounts should be looked at as a form of retirement funds. Obviously its not my primary retirement fund, but with a little bit of luck it could be. On the other hand I could lose most of it. So I'm always adding to my bankroll with the extra funds I have. it is my last account I invest in, and that's even after my basic checking account and savings. If I lose money i don't need, its not lost money. If I make money off I don't need, then I'm closer to retirement.