I'd say you'd need to do atleast 4 hours a week (so literally could be 1 hour or 30 minute sessions spread throughout the week), but make sure the revision is actually good revision so like doing exam papers/questions, blurting - stuff thats active recall not just writing notes down
I know you've said your not able to do any revision over the holidays, but is there any chance you could just try do a bit of seneca? Just do a few topics on that for a little bit? Even is its small you might still pick it up and remember it in the long run