It really depends on you and your situation. What are you getting at the moment? What subjects are you struggling at more? Do you know how/why you're struggling? How are you revising? Are you focussing on your weaker topics? Are you splitting your revision time each day into short bursts of different subjects? For example, you can spend hours and hours rereading the textbook, making notes etc. but still not get an 8/9 in science because 1) you're not actively recalling information so you can remember it and 2) you're not applying your knowledge.
There becomes a point where it doesn't matter how long you're revising for, as long as you're doing a decent amount of good revision where you're going over your weaker topics and actively answering questions. Start small (like maybe only 30 mins/1 hour a day) then gradually build it up towards exams - although if you really want a number, 2/3 hours per day nearer my exams worked out. Just so long as you're revising effectively.