If the margin equity in your account falls below a certain amount based on the amount you have borrowed, then the account is issued a margin call. (fidelity.com)
There's also no set repayment schedule as long as you maintain the required equity in the account. (investor.vanguard.com)
For options and other securities settling in one day, you must have sufficient cash or margin equity in your account when your order is placed. (fidelity.com)