I hear you, Yen. The more intense the workout will be, the longer the warmup needs to be. I just started doing 15 minute criterium races, and I'm warming up for 30 minutes, with some 1 minute intervals at race-pace. Then a 30 minute warm down afterwards. 60 minutes of warmup and warmdown for 15 minutes of racing!

My club ride takes off slow, and doesn't pick up the pace for 20-30 minutes, which also happens to be when we hit the more open roads.