Hi, I'm new here.
I have a Master's in performance & I work as a freelance professional accompanist, but I also have a full time day job that does not involve playing. I have been wrestling with these issues for a couple of years: how to use my few available hours to keep my technique up, learn new pieces, rehearse & perform, and still have a few hours left to eat, sleep, and be sociable.
I used to average 7 hours a day when I was an undergrad in music school. I know becauae I counted them obsessively. Sometimes I spent 2 of them on scales & Hanon, either with a metronome going or listening to the radio news on headphones. Take my word for it, that was not helpful.
Then I had a really wonderful teacher one semester who gave me some great advice. I went to my first lesson, and he asked me to play a scale. I ripped off 4 octaves of C major really fast, hoping he'd be impressed. Then he said, "What were you trying to do?" I said, "Uh, I just played a scale." Duh. But he meant that I should decide first what tone, dynamics, voicing, rhythm, etc. I want & hear it in my mind, then play it and also hear what came out, and always know whether or not I did what I meant to do.
I tried it and found that I could accomplish way more in a shorter time. But it is a lot harder because you mind has to be completely engaged (no more listening to the radio...

Exercises are like a hammer, or any tool. If you hit the nail right on, the right number of times, it is just right to hang the picture. If you hit it too much, it goes into the wall and the result is useless. If you miss the nail and smash your thumb, you were better off without the hammer.

It's what you use it for, and how, that counts.
I have started getting good results for memorizing by using lots of mental imagery techniques and studying scores away from the piano. There are some good books on this. It works very well, but takes a very high amount of concentration that I can't muster very often. Still, I memorized a short piece this way in an hour, then go months without touching it and still remember every note.
I have vastly improved my sightreading by reading piano trios once a week with some friends of mine. It's great fun as a social activity, yet doesn't take away from my practicing. I'm able to read at a level much closer to my technical level than before I did chamber music. Cuts out some of those hours when you are just struggling to learn notes enough to get through a new piece.
The other things that help are exercise, sleep, good diet, yoga (or anything else that reduces tension & makes you aware of your body), and learning not to take your artistic quest quite so seriously. Yes, of course it's serious, but you should find joy and fun in it, too. Always play the music, even if you sometimes miss a note. You won't be shot or put in prison for hitting a wrong note, even if you do it onstage. I once heard Menahem Pressler and the Beaux Arts Trio perform a Mozart Trio, and when Pressler got to the last chord, he completely missed it and crunched a very obvious wrong note. They still got a standing ovation, and nobody suggested that Mr. Pressler should have spent more hours practicing.
--L., amybeachfan