Personally, I think this (ie, thinking in terms of time) is the wrong way to approach practicing.
I rather think for an effective practice (for effective anything, this applies to Algebra as well) you should set realistic goals, sit at the bench and don't leave it until you are done.
But how to size a unit of work?
It doesn't have to be "learn a new piece perfectly every day".
It could be small.
It could be "practice this up to a fraction X of the intended BPM until it's OK", "learn 10 new bars", etc.
Size your work units small enough that you can do them correctly, with no mistakes, in a single session - that, I guess, beats "go haphazardly through whole page in a sloppy fashion".
Leave the bench having actually accomplished something - be it play two more bars of a piece, doing arpeggios at a couple more BPMs or perform a whole concerto, in the same way you leave your desk after having solved a math problem :)
Funny story: I once heard a very good (classical) guitarist saying that his secret to practicing was to never make mistakes.
Got some dumbfounded looks, because hey, if you never makes mistakes you don't need to practice, right?
Wrong, he then explained: he never said anything about anything else, so if his average day had to be "play TWO bars at 1/4 speed - without mistakes", so be it - we'll raise the metronome tomorrow without carrying over today's mistakes.