Only as long as you need to complete the next set, no longer. I usually rest longer between sets as the weight gets heavier. If I have more than 1 set of 15s during the earliest sessions, I'll rest 45 - 60 seconds. During the final week of the 5s, I'll rest 2 - 3 minutes or more. And I rest longer for the big lifts like squats and deads than I do for little stuff like curls and tricep extensions.
