I never worry about time. I start my next set when I feel I can give it 100%. You'll know when that is. I use to be a time junkie, but not anymore. I learned to abandon that when I started training arms and legs differently. I had a new training partner and liked to alternate muscle groups in between exercises for legs and arms. Meaning he would do barbell curls (4 sets), then tricep pushdowns (4 sets), then another bi movement, then tri moevement and so on.
I use to think the muscles wouldn't get the proper stimulation due to letting them rest this long between exercises. But I was wrong. I actually liked training like this for awhile. I don't do it all the time, but it's great for a change of pace. I got some good gains doing this for a couple of months. Especially my legs. But this proved to me that resting to long doesn't negatively effect stimulation as I thought before. Least it didn't for me.
I use to power train as well. Not necassarily powerlifting, but I was using nothing but basic exercises and powerlifting exercises for 3-4 sets and 6-8 reps. I trained like this for about a year. I took no less than a 3minute break in between sets. For the larger bodyparts I took around 5-6 minutes. It wasn't that I was timing them, I just happened to have a clock on a wall nearby and would notice from time to time how long I was taking. Of course I attribute most of my size and foundation to this period in my training more than any other.
I honestly think it's a matter of preference and just what your body is use to doing. Cutler, who I have seen train numerous times on different video's (the guy has like 10 of them out, lol) doesn't rest that long. I think he knocks the sets out with about 2 minutes rest in between. But Cutler is also known for training with "moderate" weight, for his size compared to most pros. He makes up for it by resting less between sets and using a ton of exercises in his routine. For back he does what, 7-9 exercises?
Then I've seen Ronnie train and noticed that he uses long rest periods. I noticed this two different ways. One way is, a song will just begin to play, and song you know is 4 minutes long and Ronnie will just begin his set, then by the time he starts the next set, that song will be over and another song will be playing. Then another time he was training, I think on the same tape, there just happened to be a glimpse of a clock or something right before he began his set, then another glimpse of it around his second set. A good 4 minutes had gone by, and he was training triceps. Then of course you know he rest a good long while between heavy squats and deads. But as stated, he also does this on smaller movements and bodyparts. But Ronnie, opposite of Jay, use's only 3 exercises, maybe 4, and trains twice as heavy. So he needs that long rest period.
Conclusion: I think it's wise to use long rest's between sets when training heavy, no matter what bodypart. It could also help with a 12 rep range, depending on what is heavy to you and rep range you have trained your body to train "heavy" in. I personally like longer rests between sets, but thats not to say shorter rest times don't have their place. I think shorter rest times could be beneficial when getting ready for a comp, or just for a change of pace, or possibly can be used for an intensity booster. Meaning, people use supersets, giant sets, forced reps, rest-pause, etc, well shorter rest between sets could have it's place for extra intensity. Especially for someone who is not use to training with shorter rest's between sets.
It's really up to the individual training. Everyone is different. It has been proven that bodybuilders can gain muscle using shorter rests, and it's been proven bodybuilders can gain muscle using longer rests. So, that doesnt seem to be the issue, when it comes to a "must" for growth. So, I think they have their place in other categories of training.