On my free accounts (where I show my small program by hand) I note ratings in the horse's name, which serves as a guideline for when I should start entering horses.
Es and Ws can generally be shown by the end of their first month of training (5 training sessions), Ps get shown a week or two into their second month, and so on- each descending rating adding 1-2 weeks onto when I start showing them.
On Mondays, I stick any horses placing 9th or higher the previous week back into shows. In addition to that, I check on any horses who leveled up 4 or 5 weeks ago, to check if they're ready to go back into shows (6*level bars in whatever discipline they're showing in). I show Monday- Dressage, Wednesday- XC, Friday- SJ, with a few exceptions.
Horses with a single E rating are usually trained AD, unless their other ratings are S or less. If a horse has just leveled up in SJ (and is SD in SJ) I'll enter them into XC, and vice versa. But Dressage/XC I usually wait an additional week- except for when a horse is trained in SD XC and just leveled up in SJ- but isn't ready to go back to showing in XC. SD E horses generally go back in shows they specialize in 4 weeks after they last leveled up (and Es are the only horses that get slapped in SD, for me).
Any horses that don't do well on Monday (or placed 10th-12th the previous week) take another week off showing. This also depends on ratings- if I have an EEP who didn't do well in Dressage, unless they're actually stronger in XC traits I just won't enter them in XC until after the horse starts placing in Dressage- and wait until they're placing well in XC before entering in J.
If this is confusing I understand, but it works for me!