With M1 Borrow, you will be charged interest at the end of every month. You will only pay for the line of credit you actually borrowed.
You'll receive a statement after the end of each month that displays the range of dates covered in the bill, as well as the amount of interest accrued.
The interest amount due is deducted from your cash balance if available, or will increase your amount borrowed. If you have no cash available and have already used all credit available, M1 can sell securities in your portfolio to cover your interest.