The length of time it takes to solo is proportional to how much time you spend in the air. I also am an instructor at our field. There's some that have been trying to solo for years, but they are only getting out to fly 1 or 2 times a month and only spending a half hour in the air when they are at the field. You can solo in a week if you get out there 3 or 4 days and spend a good amount of time in the air when you are there. Just like full scale, it takes a certain amount of time to get the hang of it. It all depends on how hard you and your instructor work at it.