When to revise?Watch
If you prefer the "hours" approach though, I wouldn't say more than 2, maybe 3 hours on a school night (if you are in school, I'm not sure what qualifications you're actually revising for...) because I don't know what your institution's policy is on still giving homework/extra work at the moment. I'd say really take advantage of your weekends at the moment, but don't just sit and revise, say, 6 hours straight. However many hours you do- I'd say it's up to how many you feel like doing right now- take plenty of breaks and spread it out over the day a little. Working in around 30-45 minute chunks is best for your concentration, with a 10 minute break.
For my GCSEs and A-Levels, I started revising properly around March and hammered it over the Easter holidays. Hope this helps!