I water young trees and grapes about once a week during winter, because I live in a windy area. I was told once a month would be sufficient, but I've learned (over the years) that younger ones need more frequent watering.

My well-established evergreens start to brown, if they don't receive a deep monthly watering during winter. The wind seriously robs the needles of needed moisture. I usually don't water the older deciduous trees during winter, unless it has been a particularly dry winter.