Most of the ones that hook in the back, I can just pop it open in a split second. Unless it has an unusual clip. I've even managed a few surprise unhooks where the girl didn't realize until later that the bra was unhooked (this requires the girl to be lying down or leaning, and be distracted, usually with DFK).
With non-WGs, I do notice that some of the more inhibited ones are especially wary of you unhooking their bra. I take advantage of this by going straight for the pussy instead.