Howdy
I leave at least 21 days between races for a horses fitness and condition to recover.
You want the red fitness and the green condition bars full to get best performance.
Potential increases in the first 4 races of a horses first season.
After that it may increase in varying amounts at the start of each new season up to a 5yo.
This post covers potential increases....
http://www.startersorders.com/phpBB2/viewtopic.php?f=19&t=6531