rest time is increasing as the weight slowly adds up.
I think Mr. Wendler explained it best.
I will add my two cents for whatever they are worth. I think what is important is working withing the context of your your goals and your capabilities.
If your goal is strength (and I think we all know by now how important strength is and how it affects all of the other fitness qualities), then do not turn your workouts into work capacity training by limiting your rest periods. In such case the point is to get all of your reps. The minute you start setting rest periods you are introducing a variable into the system that you possibly may not be adapted to, and anytime you do that, you are by definition entering GAS Stage 1 (which increases cortisol release beyond that which the training was already causing) and affecting your inter-set and inter-training recovery. Repeatedly done, this process brings about GAS Stage 2 and you adapt. However if you are trying to develop work capacity then controlling rest periods becomes part of your training and you train under that context. It's hard to optimally develop both simultaneously.
Keeping your training completion time, whether a TM Monday/Volume or Intensity static across from workout-to-workout, whether its 2.5 hours of 1.5 hours as dictated by your current work capacity/training loads, I think is a good idea, then at least you are developing your work capacity as your exceed previous loading capabilities. However at some point the total loading necessary to drive further strength increases outstrips your ability to maintain the same pace of the workout. At which point further attempt to keep workout-to-workout completion time static will affect your recovery and that time will have to increase.
I swear this made more sense in my head. Someone please correct me if I'm way off.
I lift at home, in my garage. Saturday morning is volume day for me (TM). The only other noticeable living thing in the garage beside myself is a semi-stray cat. No friends to make, no hot chicks to gawk at, nothing to do but lift and rest. Last Saturday my workout took 1 hour and 45 minutes from warm-up's to the end. I stretched for a good bit after that, which I didn't include in the total time.
Or you are very weak and using weights light enough to allow 2 minute rest periods. This is BBer speak and is rubbish.
I don't know what the "science" says, but many years of my own experience and observation of some very very strong men say otherwise.
Squatting 5x5 with damn close to 5RM weights will require 5-10 min rest periods for a strong lifter. Maybe 10-15 minutes for the final set or two. That right there is 30-60 mins just on squatting. Throw in bench and powercleans for 5 sets across and you are looking at about a 2 hour workout. This is pretty standard for me, and I don't even have a lifting partner much less a friend at my gym.
I think you're right. OTOH, there's nothing wrong with making a social effort, as long as it doesn't interfere with training, so you have to pick and choose your "friends" in there. I often just say "excuse me" and go sit in front of the weight for a minute with the headphones on before lifting.
I take usually 2 hrs to train (strength) and read about the old Venice Beach days, where the guys would train a couple hours, go eat, and come back to train more. They mostly didn't look too shrunken to me.