hi awesome_27 what do you exactly mean? applying for a plan and then how long can you pay your first MRF? OR, you're asking about the time from your cut-off to the time you have to pay your bill (due date)?
For the first one, it depends when would your application be approved. You will be advised accordingly.
For the second one, it is usually 20 days from your cut-off. Example: If your bill cut-off is every 10th of the month, then your due date to pay is on the 30th or end of the month.