How many hours to study per day/weekend?

Hey, I would recommend doing around 1 or 2 hours of revision a week per subject (not including past papers, which you need to do loads of as well. they are one of the best forms of revision). You could split that 30 minutes for 3 subjects each day, so 1.5 hours of revision each day during the week (with 15 minute breaks in between), and then do past papers on the weekend on what you have learnt during the week.
I too am quite a busy bee and did quite a lot during my GCSEs. I didn't give up any of my hours but I still did alright XD I guess it just depends on the person. If you know you need all the time you can get to work really hard then you should give up your activities, especially if you have an activity the night before your GCSE. Now I have to work hard for my a levels I personally have reduced my work hours to 6.5 hours (on a Saturday) so I have enough time to study during exam season. However, seeing as you've started revising now, you could probably still afford to continue all your activities seeing as a quick read through your notes and a couple of past questions the few days leading up to the exam would probably be enough if you have retained all the information.
Good luck!