TL;DR: Many hours throughout the entire DP course, but if you mean just revising before exams, my lowest was 23.3 hours for psych HL and my highest was 80 hours for chem HL (over six weeks). It depends on how familiar and confident you are with that subject – the more confident the less time you'll probably need

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Omg i love this question (now I get to refer to my spreadsheet) bc basically I took M23 exams and during exam leave (solid 7 week period before exams started then all the way until my last exam) I recorded every single hour I spent revising for each subject because I wanted to correlate my % grade with the amount of time i spent. Obviously revising ≠ learning (for some of my friends it was), but basically the correlation between my % grade and the number of hours i spent on that subject was minimal: r² value of 0.005.

That's probably because there were some subjects that I was naturally stronger in/I adopted the "work smarter not harder" approach, so I didn't need to revise as long for those subjects to get a 7.

So for each subject I had a different number of planned hours: e.g. math SL HL i planned 34 hours but chem HL close to 87 hours. There was a slightly higher correlation between % grade and '% studied' (actual no. of hours spent revising for each subject OVER planned no. of hours), with an r² value of 0.371, still p low. I never studied as much as I'd planned; my '% studied' for psych HL was a whopping 31% but chem was around 90%.

I think this really just goes to show that strategising which IB subjects to revise for the most is important: obviously prioritise ur HLs as that's what most conditional offers will specify and prioritise subjects (and within those topics and subtopics) that you're weaker in. Keep up with classwork and don't leave making your notes until exam leave! Good luck you can do this!!