This is a very good question and it can be different from person to person. However, there are some general guidelines that apply to all of us which can be helpful to everyone.
Personally, I like to start memorising music early in the day while my mind is fresh. After I get up in the morning and have a nice meal, I like to warm up and refresh the music I worked on the day before. After that, I will delve right into memorising music. This is not to say that this is the only time to memorise music.
In fact, studies have shown that if you memorise things right before you go to sleep, you will retain more information.
Your mind will assimilate what you’ve memorised as you sleep! This is not to say that either method is correct or incorrect because one size does not fit all. Some people might have their minds working best in the evening while others will be scattered and unfocused later in the day. Personally, I can work late into the night as easily as early in the day, but starting early allows me to learn more.This brings us to the subject of how much you can attempt to memorise during the course of a day. You really can’t overload yourself with too much information at once. For example, if you were moving your household, if you attempted to take all the boxes and furniture at once without stopping, you would become completely exhausted and probably wouldn’t finish the job. However, if you were to take your time and take frequent breaks, you would be much more productive in the long run.
The same thing is true for your mental work. Learning to pace yourself and work a little bit at a time is much more beneficial than trying to cram memorising all into one session.
I recommend working a bit on memorisation and then taking a break by doing something else in your practice.
Or, you could do something completely different! Keep coming back to memorising your music throughout the day and you will be amazed at how much more you are able to retain over time. Each time you sit down, first you refresh what you have learned previously. Then, forge forward learning more of the score each practice session. If you have the energy, refresh things just before going to bed, and your mind will keep assimilating the music as you sleep!
Try out these suggestions in your practice and see how this increases your productivity.