How big of a bankroll should you have?

Ok... I'm new to this, could someone help me out please. I recently read "Small Stakes Hold'em: Winning Big With Expert Play." I have played entirely online, specifically partypoker. My results have varied greatly. Playing only two 3/6 tables, I have made as much as $400+ in two hours. Yet I'll turn around and lose all of that plus some during the span of several days. I understand that because poker is gambling, it will have "swings" for better or worse, irrelevant of one's playing style. So my question is this: assuming correct playing style and 3/6 tables online, how big can I expect these swings to be? Or, said another way, at what point can I know that it is my play, not my luck, that has caused me to lose (or gain)? How big of a bankroll is needed to compensate for luck?