An accomplished archer could do it pretty quickly. Okay one round may be a bit too quick. In reality it depends on where you had the string. If it was kept on the bow (the top loop slides down the length of the bow a few inches and the whole thing hangs limp) it could be done very quickly. If you'd taken it off to keep dry then you first have to loop it over one end, tighten it, then brace the bow around your leg and slide the other loop over the top. A bit more fiddley. Depending on how much you want to game it, you could even ask for a dexterity check to determine how long it takes if the character is trying to do it under pressure. 2 rounds max if its the former example, 3-4 rounds if the latter and you feel like making life difficult 
I think the main thing is to make the players prepare in advance or really be punished if they get ambushed without having weapons readied.
I think the main thing is to make the players prepare in advance or really be punished if they get ambushed without having weapons readied.
