This may also depend on what you want to achieve when you open them. As George said it needs to be right to do a full inspection, even then it depends how quick you are, last year was my first and I was slow initially and even when it was decent temperature they started to lose their patience with me after 15 minutes.... !
(I have probably halved that since for a full inspection of all brood) Having said that I was eager back then to look at every comb and find the queen every time, now I might only be 3 minutes or so if I just want to check for eggs/brood/stores etc.
I opened mine on Sunday afternoon, 3 degrees here at the time, but I was just checking fondant and had the crown board off no more than 30 seconds and the insulation for less than 10 seconds.
Fair to say the bees will let you know if its too cold, but my beginners course told me it needs to be 16 degrees minimum for a full brood inspection, if its windy I normally stand up wind so as to shelter them a bit and sometimes use manipulation cloths.
Did that help....... got too much spare time at work.......... or shouldnt log on during the day !
Pete D