I've done reviews for 76 scientific journals by now, and am getting a little restless when being given a deadline of 14 days or even less (a week recently!) as deadline to return my review. Obviously one can get an extension on request, but my point is this: For me such a deadline carries the message that my time is not valuable or that I'm sitting here twiddling thumbs, eagerly scanning my inbox for the next review request. This is not the case :). I regularly decline such requests now.
Happy New Year!