I was told by my coach to take no more than 2 minutes of rest between sets. I assumed this to mean after the weights had been changed out.
What I did was set a timer to beep every three minutes, so the weight changing time was included in the between-set time. I simply forget to time individual rest intervals and take too long if I don't time them. Hence the ongoing three minute interval timer--it only requires me to start it once and stop when I'm done.