I think I'm missing something here. As a learner you have no chance to decide on the reward to gift your mentor unless it's a free course, the mentor will decides on what he/she will accept and you may either chose to agree or disagree. But as you requested, I assume it to be a free class and in that regard I will say it depends on individual but regardless, the best reward for mentorship is gratitude and appreciation. Although everyone value gratitude in different ways, to some it maybe material gift or cash while some are satisfied with a mere thank you. So you need to figure out which one the mentor prefer and you can start from there. As for frequency, it will depend on how long you both agreed with each other but it is also good to keep up with the appreciation even after your learning period.

The important thing is to make sure your mentor knows how much you value their help and support.