As someone who always takes the window if possible, I try to get up when the person next to me does, even if I don't need to just yet. (I actually try to race and use a different restroom and try to get back before my seatmate.)

I can't think of any time I've needed to get up when my seatmate was sleeping. If I need to get out, I try to wait for an opportune time, such as when I see that person's movie ending.

Politeness is key but I certainly don't think getting up every 4 hours or so is unreasonable. I'd never try to climb over.

Seth