1. How long do you practice? I've found that, as I'm getting a little more fluent at the keyboard, and thus liking it more, that at least 2 hours can go by before I notice. If I'm having a bad day, of course, I notice. Sometimes I wonder if my returns aren't diminishing after a certain point, however.
I think you can overdo practising on one piece for hours a day for days one end. It's probably better to do something else in between. I'l like to know what concert pianists do if they are learning a (new) work for a concert.
2. How many pieces do you have going at once? Up to now, I've had one polyphonic (i.e. Bach, Pachelbel), and one "homophonic?" chordal (i.e. Burgmuller, easy Haydn sonatas) going at the same time. More than that, and I can't remember what I'm working on.
I always have dozens of pieces in the stew for recording in the near of non-so near future. Maybe that is not the right way and one should work on at most 2-4 pieces at a time.
3. Do you aim for perfection of a piece before you move on?.
You would not move on a lot if you did that
Seems the better you get, the more you realize how far you still are removed from perfection. I find that frustrating. But ok, as long as it still gets better, there is hope. And it's better not to push too far.
My lute teacher, who seemed like an outstanding pedagogue, said that, especially in the beginning years, it was ok to move on from a piece before it was perfect, because after a couple of years of playing you're simply so much better that you'll play those old pieces perfectly with far less effort that you would have expended in the first place. I have definitely found this to be true in non-musical disciplines. Also, I've gone back to pieces I agonized over years ago (before I got serious about the piano), such as the Petzold minuet in G, and I can now, without having looked at it in the interim years, play it perfectly at sight (or at least aided by the memory I have of it).
Indeed, if you get back to a piece, after having had a reasonable grasp of it earlier, you seem to get a lot of things for free. Alone for that reason it pays to put a piece aside now and then, also to avoid it getting stale.
Interesting you play(ed) the lute. Such a noble instrument, I can't understand why it has gone out of fashion and nobody writes for it anymore. Or does anybody still ? The lute suites by Weiss and Dowland are exquisite.
4. Would you consider a piece "learned" if you can play it "perfectly" but not at the designated tempo?
Hell no ! That'w when the work starts.... But it is a very good basis, and it's better to do it that way than to favour tempo over accuracy. Now only if I could put my money where my mouth is