I'm building my bankroll playing at the .50/1 limit tables on Party Poker, usually I crush these low limit games but just like everyone else I go through periods, sometimes amazingly long periods, where I either can't hit a flop or can't see a hand.

As a rule of thumb how long should I play for or how much of my bankroll should I go through with out winning before I should take a break?

How long are we talking... how many tables are you playing at a time and how much money is in your poker bankroll?

I've been playing 3 or 4 tables at a time with a bankroll of 250.

I would stop and take a break when it starts to aggravate you, and influences your gameplay.
You can also slow down and play only one table at a time for a bit.