What a great question! It's not surprising that you might ask this, since the time to study varies greatly. The short answer is "it depends." You first want to ask yourself several questions. When I have something really difficult to remember or study, when is the best time for me to do this? Am I a morning person? Do I like to work on things late at night? Do I like full light or soft lighting? Can I sit and study for long periods, or do I need to move around after 20-30 min.?
 
The best time to study really depends upon your own learning style and body rhythms. For instance, my body always slows down between 12:30 pm and 1:30 pm --- no matter what; then, I'm always ready to think or write from 9 pm to midnight. My husband loves getting up at 4:30 am and working quietly until 6:30-7 am. Some people need soft music, ex. Gary Lamb or Mozart, to work by while others don't tolerate any noise at all.
 
So, you can see. Everyone is different. May I suggest studying for short periods of time (20-30 min.) but frequently during the day and or evening for better comprehension? Bottom line --- figure out when you think best. But, ALWAYS remember to leave the TV off, as well as the contemporary music.