If it's a long ride, I'll want to eat first, then if it's winter I need to work out much to wear, putting booties on and taking my bike from the sixth floor to the first takes a while too. I'll do a last minute check too, make sure i haven't forgotten the wheels and that I tightened things back up. Decide what tools to pack, what sunglasses, no of spare tubes etc.
It's never taken more than 40 minutes and sometimes only 2-3 minutes, but if I'm planning to meet others, I usually organise it so that I have an hour between lectures and meeting for the ride as I have to include 10 minutes to get to and from home too.