Maybe, if I spend the whole of the summer studying, I won't die of exhaustion before I can take my OWLs.

Ancient Runes - 3 hours, Saturdays & Sundays
Arithmancy - 3 hours, Saturdays & Sundays
Astronomy - not that important? What am I going to do with it? 1 hour, Sundays
Charms - 3 hours, Mondays, Wednesdays & Fridays.
Defense Against the Dark Arts - 3 hours, Tuesdays, Thursdays & Sundays
Herbology - 3 hours, Mondays, Wednesdays & Fridays
History of Magic - same as Astronomy, probably
Potions - 3 hours, Tuesdays, Thursdays & Saturdays
Transfiguration - 3 hours, Mondays, Wednesdays & Fridays

Twelve hours of studying in a day is probably fine.

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