3rd., 4th., or 5th up would be about peek fitness for most horses except those under trainers well known for preparing forwardly fit horses 1st or 2nd up.

I've never liked horses that have backed up after 7 days, especially if their last run was on a wet track. It takes the stuffing out of them. 10 to 21 days seems a good rest period without losing fitness. 4 to 5yr. olds seem to do most of the winning and I think about $5.60 or less SP account's for most of the winners.