I read somewhere (on exrx.net) you should do >90 seconds rest for power lifting, 30-90 seconds for bodybuilding, and <30 seconds for endurance training. I cannot find that specific citation.
Read 'Muscular Endurance' at http://exrx.net/WeightTraining/Tidbits.html
and the second to last dark bullet at http://exrx.net/WeightTraining/Guidelines.html
Always keeping a specific time between sets helps me because my muscles have the exact same amount of rest every workout so I expect to life the same weight the same amount. You're probably resting so long that you're muscles have time to fully recover an the extra time isn't really helpful. See 'Strength Recovery' at http://exrx.net/WeightTraining/Tidbits.html
. If you're pressed for time, you'll probably do alright keeping your rest time down to 2 minutes.
I haven't seen any evidence that you shouldn't just choose a time you want to take between sets and just stick with it. Yeah, you'll probably have to drop a lot of weight, which is disappointing, but you should get adequate results (assuming you've read and understand the links above and fit your workout to your goals.)