Some bows take more time, like sinew-backed recurves and such, but I've tracked the time on 'standard' fiberglass bows, backed bows, and selfbows, and they all take me roughly 20 hours... give or take.
Cutting out and prepping a riser piece for say, a one piece longbow takes me about a half hour... though I've done it in half that time in an emergency and near state of panic :^)