I know you probably know this and it's not what you're asking, but don't just use time as an indication of how much to revise. Don't just say "i'll do 1 hour of this tonight". Make specific targets like "I need to finish this homework and understand this topic".
That said, I would say per night you should be doing a minimum of 30 minutes from Sept-March. This isn't per subject, this is just overall. Sometimes you will need to do 2+ hours, sometimes if you're on top of your stuff you only need 30 minutes to just make some extra notes. It's about what you need to do not necessarily how much time. I would argue the only subject where you can use time alone to justify effort is maths practice questions, especially if they're the same type of questions (i.e. i'll do as many of these integration questions in 1 hour).
How can you tell if you're doing enough work per night? Well if you're able to keep up in lessons and not find the subject too challenging, you're probably doing fine. IMO the best tip to use throughout the year is to revise 5-10 mins just before your next lesson of whatever you did last lesson. If you go into a lesson with what you did last lesson fresh in mind, you will find the a-level so much easier than others who didn't.
After March, you should be doing a minimum of 1 hour per day, sometimes 4+ hours making very busy evenings...
I wrote a pdf a few years ago after I finished my a-levels (3A*s, 94% average) with some advice, check my profile for the link