Depends on what level horse it is.
If it is a novice horse still trying to find the perfect jumping technique, then small jumps maybe 2-3 times a week (all sorts of exercises from gridwork to just one jump to doubles to free jumping etc.), spread evenly along the week doing flatwork/hacking in between. If it is a fairly experienced horse then I would do only 1-2 times per week, but at least one of them would be jumping nearly to the maximum heights (i.e. over the competition limit), the other one maybe a gridwork exercises.
Flatwork is the key to jumping. I know many SJers hate flatwork with passion, but it really is an important part of showjumping. The better you do on the flat, the better you will do on the jumps, both you and the horse.
Nina x