Depends if you are looking for a speed exercise or a (speed-)endurance exercise.
In pure USRPT you would use 15s rest for 25’s and 20s rest for 50’s and more.
I usually do (SCM):
25’s on 35s
50’s 200 pace on 1:00
50’s 400 pace on 55/1:00
75’s 400 pace on 1:10/1:15
100’s 400 pace on 1:30
training for a 50 is mostly technical, work on turns, break out, starts, under water kicks
Swimming exercises:
– 25’s MAX speed with start, on 3/4/5 mins
– 25’s split 50 / 2, on 2:00
– 6-7s max efforts on cord (with resistance)
– max sprints with fins and paddles
– running turns: take a few running steps on deck, dive in, take 3/4/5 strokes and do turn at overspeed