I can't answer this one very well as I get distracted all through my practise sessions with my phone and playing with my cats but I try to average about half an hour of practisting pieces and about 10 minutes of scales to warm up. If I have a quiet day I''ll try and get an hour in.
To begin with I had one or two "main" pieces I practised on that was like a proper bit of music, one or two pages of something like Hall of the Mountain King or Fur Elise (I know, I know) which would take me a few weeks to learn and perfect and then I would try and keep my favourites of those up to speed so I had a rudimentary repertoire. But at the same time I would have two or three shorter pieces that were more about technique from the lesson book and I would get them down each week then move on to another set of pieces.
Learn to sight read and how to read ahead. Eventually stuff is going to get complicated enough that you can't memorise it all. I got used to tempo with a metronome, I like this one because it's small and the ticking isn't so loud that it's going to bother other people but you can't set it to "bing" at the start of a new bar. I don't need it as much now but it definitely helped at the start.
The exams helped focus my lessons and I improved a lot faster with them and it made me learn the theory and technical bits to playing piano but I don't know if that was just my tutor teaching better when we working towards them.

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