So much of your mosquito experience will depend on where you go and where you stay. If you are travelling in rainier, more tropical parts of the islands, there are likely to be mosquitos around.
But, we always stay in Kihei, which is very dry and (while I get eaten alive when mosquitos are around anywhere else) in Maui I never even think about mosquitos or bug spray at any time.
Stay somewhere dry, and forget the deet!