There's no actual rule on how often you seed, as long as you maintain an acceptable ratio. The only problem, as buchno notes, is that you won't seed nearly as much. In fact, it's possible you won't get much, if any, seed action at all.
Why? Because most — in fact, just about all — of the torrents on BakaBT are heavily overseeded. Leaving your PC running gives you a lot more chances to compete for peers to seed to.
Why can't you leave your laptop running overnight..? If you're worried about wear on your HD, you should be aware that spinup/spindown (when you turn your PC on/off, or when you let it hibernate) is actually far more wearing on an HD than spinning constantly.
