That depends on, which game you are talking about, and the whole situation. Tournaments have more variance than cash games, and large field MTTs have much more variance than single table SnGs. It also depends on, what the purpose of the bankroll is? Are you for instance willing to move down, if you hit a rough path, or it is important for you to be able to stay at the limit, you have reached, without a significant risk of going broke?
If you are trying to grow a small bankroll, it often makes sense to be quite aggressive, since the worst, that can happen, is, that an attempt to move up fail, and you have to move back down again. But if you have already reached the level, where you want to be, and perhaps you make regular withdrawals, then moving down kind of sucks, so you should be more conservative with your bankroll.
It also matter, if you always play the same limit, or if you mix up limits. In the latter case your average buyin is more important than the highest buyin, at least if they are somewhat close to each other. Like if you play $2,2 - $5,5 games with an average of $3,5, then your variance will not be much larger than, if all your games were for $3,5.
Finally many people use the word "bankroll", but they play in a recreational way, where its more reasonable to talk about a budget. So it also matter, if you just want to be able to play until the next paycheck, or if this is really the money, you have set aside for poker, and if you lose it, you will need to stop playing.
