Establishing a default due date for when the invoice you send out needs to be paid by
You have the ability to set this up by going to Settings > Admin > Payments & Finances > Invoices.
From this section, you can enter the "Invoice Default Payment Days for Customer Invoices" as well as "Invoice Default Payment Days for Business Invoices".
For example, if you would like to ensure that the invoices get paid within 30 days, just input 30 into those fields.
Please note that the default payment days inputted will also affect the automatically generated repeating invoices, if you have these set up of course.