I have approximately 2 hours of music memorized. I am polishing the 1st hour by performing it as often as possible at different venues. How I learn a piece of music is to immediately translate what is on the sheet to my fingers. I then spend most of my time considering how the fingers move. I make reference to the score to check to make sure I am fingering the music correctly. Thus, from the beginning of working on a new piece I am already memorizing it. Once I can play through it in its entirety from memory I will go back and focus on specific sections period from time to time my memory does slip and they may forget a passage or a chord but I simply go back to the score and replace it.
A while back I had a regular gig in a wine bar. I would play for approximately one hour take a 10 minute break and then play the remaining hour of music.
For me it's more about remembering the hand positions and what is going on with the right and left hand that it is about memorizing the score.