You can pick the most suitable period for you by yourself.
It can be NET 7, 15, 30.
There are available only NET 15 and NET 30 by default.
After you receive 2 payments on NET 15 basis you will be able to pick NET 7 period.
On NET 7 basis reqests are created on every Monday for the previous 7 days.
On NET 15  basis reqests are created on every second Monday for the previous 14 days.
On NET 30 basis reqest is created on the every fourth Monday for the previos 4 weeks.