I really like the pieces I chose to play now, so I can focus on them for an hour or two. But in the beginning it is one hand alone, very slowly so I make no mistakes. Any mistake I make twice, I circle it, or write the note name next to it if it is above or below the staff. Or write the flat or sharp next to the note I miss.in case I have forgotten it from the key signature. I make photocopies now of stuff I learn, so this messy stuff doesn't mess up the book.
Once I've got one hand alone down without mistakes, a whole passage, then I slow down again and try to put the hands together. By this time I've started sort of memorizing it so I don't have to watch the page all tha closely. Then when passages are mistake free, I start speeding up.
I find aerobic exercises (walking, exercycle, bicycle) and strength training (some pilates, some of Army daily dozen) build up general strength and stamina, something no music teacher ever taught me. The teachers should have taught me the taught me the conditioning. I learned the benefits in the Army after I was 18 and had wasted 6 years making wimpy low pressure noises with my bassoon (but still got in the TMEA All-state band). School gym was useless, all they did was ignore me when I couldn't reach their standard. They had no plan for kids that could't do one situp or one chin-up. The Army had remedial training that was actually doable and improved my strength and condition, instead of being impossible and making me feel incompetent.
Now at the start of most practice sessions I play 3 Scott Joplin rags, which are very physical, and stretch my hands out so I do octave jumps without thinking too hard. When I was a beginner my mother made me do Schmitt exercises in the Schirmer book, which are especially good for building up the fourth and fifth fingers. Then the paid teacher made me do Edne Mae Berman exercises at the beginning of each practice, then Czerny book one. I don't do these exercises anymore, I have enough control to play things I like, but I did exercise book practice for the teacher for 6 years.
At my age, about 2 hours practice is my limit, or my tendons start causing pain. but I'm 63 years old. A teenager or twenty year old should be able to build up to 2 hours practice easily, with three to six occuring when you get advanced repretoire in your sixth to eighth year. What I actually did was one hour a day of piano and 1/2 an hour of bassoon, until I quit the piano age 16 to concentrate on the bassoon loaned to me from the school band. Bassoon involved practicing with girls, and going on road trips in a bus! In band we were playing stuff like Bernstein and Grainger suites and Shostakovich symphonies, besides the circus marches and pop songs for football games. Piano now fits my days alone better. On piano now I'm finishing up Pictures at an Exhibition and am starting on the original 2 piano arrangement of Rhapsody in Blue. On Hammond organ I'm working on JS Bach Passacaglia & Fugue in C min These are all my favorite pieces, so I don't have any trouble concentrating on them when outdoor projects of summer don't get in the way or practice.