For students that attend elementary, middle and high schools with a traditional school year calendar, you should continue to learn during the summer months to avoid "Summer Learning Loss". Summer Learning Loss is real. Take a look at the infographic on the left to read a few statistics about it.
This is something that requires commitment to learning (despite not being in school) and doesn't have to consume a huge chunk of time from week to week. This infographic suggests 2-3 hours per week needed to prevent any learning loss. I would lean toward a minimum of 4 - 6 hours (not a lot out of a 7 day week of 168 hours) and not put a limit on it.
When anyone is engaged/enjoying what they are doing, time goes out the window and learning is unknowingly taking place. The beauty of Summer Learning is that (outside of your required school work) you can take control of what you learn and how you learn it. Set a schedule of when and how long you plan to work on improving skills and learning something new, and stuck to it. By implementing and carrying out a Summer Learning plan, students will enjoy a sense of pride and accomplishment as well as a boost in self-esteem that will contribute to a more enjoyable school year.