Depends on how much you want to succeed with your horse. You also have to have a good steady timeline for yourself and your horse. Know your horse and yourself and work from there. I would recommend when ever you have time if your wanting to go high. But remember your horse might not be able to handle that. Good luck :)
I ride an hour and half every week but ride different horses with different abilities one some of the horses I do be on I’m do be on showjumpers other weeks hunters and then just lazy schooling horses I find it just really depends on you and the horse your on I’m able to jump between 1’20 and 1’40 and the showjumpers but only about 90 on the schooling horses
I'd say have a baseline of 3-4 30/45 minutes work sessions (focused productive schooling with 5-10 minutes walking loose either before and/or after if you have time) a week and max 6 rides a week depending. Good to experiment with what your horse needs and can cope with depending on your own schedule.
This is an unanswerable question in my opinion. It depends on the horse, rider, and how they work together. The definition of successful is also different for everyone, so one may think that Grand Prix jumping is successful whereas others may just want to be happy solely with their progress.
The fastest I had ever improved and moved through levels was when I rode three times a week. (And it wasn't on my own horse). Now I ride my own horse once a week and it's still good but I wish I could ride more often