I regularly spend more than 8 hours a day at a keyboard but hand fatigue is not something I experience at all. Back, neck - sometimes. Hands? Never. Though I have put a lot of thought into ergonomics - desk, chair, staggered layout custom split keyboard, nice mouse (not vertical yet but I'll probably switch soon). May I suggest you explore ergonomic equipment if you're not familiar with it?
Back when I tried playing guitar a bit I actually liked taking short breaks from work to play. It gave me something else to focus and come back to a problem with fresher mind.
WTF is this graph?