Here's my thoughts.I don't think you can ever quantify these things. I am also not convinced hands separate is the most efficient way to learn in the beginning. People like to have set numbers or methods like an "this is how many times you should do something to definitely ensure you've done it enough times" but it's such an inefficient way, you need to think for yourself and learn your own self when you've practiced something enough that it no longer fails.I think you should almost always start hands together, unless there is a certain passage for one hand that you cannot do. Everybody wants to take their absolute time when learning the piece, and you end up getting bulked down too early with the little details and never actually finish anything.The best idea is to jump in at a comfortable speed, with aim to push tempo ASAP and iron out the creases as you go along. Jump between fast and slow practice if you feel like you're losing control at any areas, but let it just be to re prep you for going fast again - like warming up before a run.Before you know it you've done 90% of the piece and you can then spend your time on those difficult sections to tidy them up. Logically you should start with those difficult sections, but for learning pianists it can sometimes be difficult to identify those when looking at a new piece.So yeah optimum, is from the very beginning, see what doesn't work, and if you can't play slowly hands together at this stage then consider Hands separate. For difficult rhythms, take that section out of the piece and practice it until you full understand where the note placement is.
The best idea is to jump in at a comfortable speed, with aim to push tempo ASAP and iron out the creases as you go along. Jump between fast and slow practice if you feel like you're losing control at any areas, but let it just be to re prep you for going fast again - like warming up before a run.Before you know it you've done 90% of the piece and you can then spend your time on those difficult sections to tidy them up. Logically you should start with those difficult sections, but for learning pianists it can sometimes be difficult to identify those when looking at a new piece.
Even with Bach?