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.
Whatever works for you and for your child, as long as it's consistent. So, some people's routine is that they do homework as soon as they get home and it's checked before t.v. or computer or whatever. Other kids need some "down time" after school so there may be 15 - 20 minutes of freedom (nap, t.v., computer, phone calls) and then homework begins. The tricky part about the second options is that you must be consistent with the agreed upon duration of free time; it cannot be extended to delay homework.
Of course, the homework space is free and clear of distractions (including family traffic and conversation) and has all the materials needed to complete homework. A desk or table with ample lighting, pens/pencils, crayons, paper and school or age appropriate tools (calculator, ruler, highlighter) works well. Good luck!