Well the entire series is 3 years across 54 books, which makes it 1 year for every 18 books, breaking down to less than one month per book. That's just averaging out though, as the amount of time between books varies dramatically.
In #2 we're told Tobias has been trapped as a hawk for a week. By #3 it's been a month. Later books are clearly several weeks apart at least, if not longer, and others (eg. #20-22) have no time gap at all.
However, with the exception of Megamorphs#1 there is never a clear indication of when they have time off school. There are times we can speculate that they have time off (some books mention going to the beach or other activities that tend to be more of a summer thing) but from #25 onwards even if they aren't in school they might just be getting the Chee to cover for them. So either their summer breaks happen between books, or they simply don't refer to them.