If you're that pressed for time do some drop set training so that your first couple of sets are still heavy. For example, do a decent warm up then hit a few heavy sets of squats for 5-8 reps. After a couple of those sets you've hit the fast twitch fibres to a decent extent, so start pulling plates and going to failure. After a couple of drops your quads will be destroyed and you can probaby be out in less than half an hour.