How many hours of revision do I need to do to get decent grades?

It's impossible to give a number of hours required answer. If you understood all the material, then you won't need a massive amount of revision. If you didn't fully understand it, and need to 'learn' it, then it will take a lot more time.

My approach is to go through the material to revise and make notes that omit everything that is obvious to me. If there is too much, then I'd go back and cover the applicable areas in depth - it's a sign that I didn't fully understand it IMO. If you manage to finish everything before the exam, then you have good notes to go over closer to it, and some free time that I would spend doing past papers.

an hour per lesson they usually say but it depends on the students and teaching so I cannot quantify anything

That's what they say at my college, but I'm not sure if that's enough to secure me decent grades.

Well, then if you feel that's enough than just increase it. It is just the way it is so don't feel disheartened.

Like I said, everyone is different and so is the standard of "decent" to each person, which is why it is up to you to figure out by trial and error what amount of time you should spend. As in to say, the 1 hour of private study to accompany each lesson was mere a guideline so don't be unwilling to veer from it but fortunately your in the more than an hour side so dw u will get more than decent grades XD