probably wouldnt leave a bag on a plant not sure why you would to begin with, most have a good idea of about what time of year they suspect their plants to start flowering and just visit there plants a couple of times a week till they are sure no males are around.
and while it may be a little more difficult you can usually tell sex by preflowers well before any plants start to flower but its still a good idea to visit atleast once a week at the start of flowering incase you have a hermie that may need to be removed.