This morning I had roughly 90 minutes to do a hard set of intervals (3x2'; 3x1'; 3x30"). Since the actual work time was so short and the efforts so hard, I needed a decent warm up. I spun very easily for the first 10 minutes, then gradually increased till I started to break a sweat (another 10 min or so). Did the intervals and then cool down was essentially a reverse of the warm up.
Rule of thumb: the shorter the event/workout, the longer the warmup.