How much time should one dedicate to Islamic studies during school, weekends (having more free time) and summer break, dividing it between learning, memorization, and revision? What methods help one understand, memorize, and retain knowledge long-term without forgetting it after days or weeks?

Ustadh AbdulAziz al-Haqqan

This needs a long answer. All in all schedule yourself. If you are able to do 2 to 3 hours, this is good in sha'a Allah. When you have time you can do more in sha'a Allah.