Difficult to say. It depends on your objectives and the frequency of study. I would say a big test at the end of each term;eg Christmas, Spring and summer.
You could also give small tests at the end of a study block. EG after a few weeks work on past tenses, a small test to evaluate progress.
I think it's more important to give teenagers a love of learning English. I'm careful about how many tests I give. I think there are other ways to check their progress.