During 15s, doing flat bench - rep goal is between 30 and 50 reps. John Doe does his warmups then gets ready to do his working set, which is using 185 lbs. He gets set up and begins, doing 17 reps his first shot - I always have them stop 1-2 reps away from failure. You can tell when they are getting close to failure because the bar slows down quite a bit. So anyway, he racks the bar, I have him take 15 deep breaths - usually this is 30-45 seconds - then once he is ready, he begins again. This time he gets 10 reps before he stops - rests for another 15 deep breaths then starts again - gets 6 more reps. I have him stop and I note it like this: 185 lbs x 17, 10, 6
I only ever have them do it in three separate attempts.
This might be something you might like to try out. You do get through your sets a lot faster than you would using conventional sets. I will warn you though that during the 5s, you will definitely need longer than 30-45 seconds between attempts. I let them do 20-25 deep breaths during the 5s instead of 15 deep breaths. Yes, this keeps them closer to failure than using standard sets x reps.
Why not keep volume constant? Well I ask instead, why keep volume constant? There isn't a lot of reason to keep volume constant. We are trying to continually add weight to the bar and become stronger over time. It is very difficult to keep adding weight to the bar if you are obsessed with keeping the total number of reps the same. Besides which, as your load goes up, so does the workload, so even if you drop volume your workload is still going up.