Both of my dc attend a VA, but dd11 has way more flexibility with her schedule and with how much work she completes in a day.
I have tried both setting time limits and just working until dd11 is finished. When there were no limits set, it felt like she spent way too long on subjects. She is a perfectionist and some things were just taking way longer than they should have. We got through more material when I set a schedule with time limits, but dd said she felt way more stressed with the plotted out schedule and trying to make sure she was done by the end of the time scheduled.
So, now we are using a list of things to get done by 2:30. She has sports three afternoons a week and then she likes to go play (we live in a neighborhood full of kids, and she likes to play when they get home from PS or when she is home from practice.) This is a huge motivator. She is responsible for getting assigned work done and managing her time. So far this is working. Whatever doesn't get done on the list becomes homework.
However, I do block out an hour every evening for "homework." If everything is done for the day, then I use homework time for enrichment or study time (vocabulary flash cards, science vocabulary, etc.) or just extra pleasure reading time, logic puzzles or other quiet activity- her choice.
Dd15 has set due dates for assignments and work, so she has to get it done no matter what. This has been great for her learning to prioritize and manage her time.