2-3 sets. Intermediate + advanced 400/400h athletes should have no problem with 3. Breaks between are @7-8min. As bounds get longer, the runs get shorter. Running A’s get longer, but only as much as I feel that the athlete can maintain form. The run segment begins at@75% and as Charlie noted, get proportionally faster as the distance shortens and fitness is gained. I also throw in ab work at the end of the running A’s.
Hope that this helps.