as long as you keep the work fun for the horse and keep the horse entertained then as much as you like, once a day, once every other day, whatever is good for you, just work up the horses fitness, start out with one a week for 2 weeks, twice a week for 2 weeks, 3 times a week for two weeks, until you build up to everyday. keep the workouts short, mostly you shouldn't be spending time on just riding, it should be more working up a bond with your horse instead of riding, do TONS of groundwork. spend time on groundwork everyday you possibly can, work on leading, she should be able to follow you forward backward and side to side with a lead with slack in it, and she should trust you a lot.
if you want to know something else or have any questions or comments then please email me back. i'd be happy to help you with anything: firstname.lastname@example.org