I got one lesson a week for a long time (with year breaks in between my riding because my mother would get tired of driving me to the barn and this includes when I rode for one of the best eventing trainers in the country for free as a working student). As long as you pay close attention and really work on remembering and putting into action everything they tell you, you will be FINE!
Maybe work at the barn over the summer for more rides/lease so you can really get some work in then! Barn chores for lessons are awesome. Like I said, I was a working student at one point. 2 days, 4 hours a day, for a 1 hour lesson once a week. It was ridiculous but it was well worth it and I learned more from that trainer in 3 months than any other trainer (besides my horse) that I've had.
Also if you want to do work OUTSIDE of the barn to help your riding. Running is great, I try to run 2/3 times a week and I can usually manage a 4/5 mile run when I'm not sick/injured! Squats are good if you want to jump. Look into rider exercise books there should be some helpful stuff out there.
I've also, returning to Dressage and remembering like nothing, taken up watching videos on youtube and trying to learn from those as far as positioning for myself. Not the easiest way to learn but it keeps me focused on what I need to work on and where I want to be one day!