Reviews are due 3 weeks from when they are assigned. The system automatically generates reminders so if you are going to be later than 3 weeks, email JR (firstname.lastname@example.org m) so that he knows. EF is very understanding, especially since the assignment has to ship internationally for you.
Ah, I just read another post which answers my question about ordering the assignments at the same time.
As a reminder, buy-out and free assignments will all have the same due date when you place the order, so take that into consideration with how many you plan on buying. And yes, buy-out assignments are automatically accepted.