May be is a silly question for someone but not for me.
How much practice volume do you advice to take for a week? or How do you know you have mastered a falseta or new concept and you can move into another item?

I know modern education advices to take several tasks at the same time, but I've found that doesn't work for me.

I don't like giving specific amounts of time because it's really so different for each person, not to mention that not everyone has all day to practice. But I agree that when learning something new it's good to stick with it until you know it (it's also hard for me to stop when I'm learning). What 'knowing it' is is a different question. For me, it's knowing what the notes are and what the rhythm is. You can't always sit with something new until you're able to play it well (sometimes this takes days or weeks or years), but getting to the point where you won't have to re-learn it next time you sit down. For a bigger piece you may not able to learn the whole thing in one sitting, so you choose a manageable chunk and work with it until you know that. That chunk may go into the 'practice' phase while the rest of the piece is still in the 'learning it' phase. And you keep going until you 'know' the whole piece.
Also - I think it's more than ok to have times when instead of practicing every day you are learning new material. It's fun and inspiring and good for other parts of you as a musician to learn new material, so if that eats into your normal 'practice' time I think there's not much harm done.
The more material you have, the more time you need to spend on 'maintenance,' so it's good to have a routine you go through a couple of times a week where you play everything you know just to keep it under your fingers, but that's separate from what I think of as 'learning' or 'practicing'
Not sure how helpful this was....
Cheers,
Kai