I generally try to keep it open only as long as needed. Weather, temp, windy, etc. all have an effect and its still pretty cold in the Pacific NW. We had our first few really decent days over this last weekend in the mid 60's that you would have also seen up north there. Mostly though, I work through in my head what I need to do in each hive before I do anything ..(I keep a hive diary and make notes from previous visits of what I did and what needs done on the next inspection), get my equipment, feed, patty's, or whatever all sorted, before I start. That way I keep the hive open the least amount of time. First thing I do though, is just observe from the outside for a while....I don't go in too deep unless something tells me I need to.