It depends on the horse and what he needs to improve in so let's assume it's a 4 year old that is as good on the ground as ridden.
I do three days a week from the ground, two ridden, one doing liberty work and one day break.
If the horse needs more work riding I will do 4 ridden 1 ground 1 liberty 1 relax. And increase ground and liberty and decrease riding.
Whenever he seems really tired we give him an extra day off, in general, this is the week For my 4 year old:
Monday: 1 hour ride (jumping)
Tuesday: 1hour ride(flat)
Wednesday: 1 hour ride(flat)
Thursday: day off
Friday: 1hour ride (jumping)
Sunday:30 minute ride (flat)
For a 4-5 years old I'd say 35-40 minutes sessions four times a week, a lunge session, and a day when he takes some rest and only goes to the walker or paddock!
For mine who's 7 years old and does show jumping:
* Monday: lunge and walker
* Tuesday: ground work
* Wednesday: Ground work or jumping
* Thursday: trotting outside
* Friday: Ground work with walker
* Saturday: Ground poles and stretching exercices
* Sunday: rest and walker