Non Federal Student loans go into default as soon as you miss just one payment and if it's a Federal Student Loan then it won't go into default until 270 days after your missed payment.
If you don't make anymore payments before those 270 days then your Federal Student Loan will be in default.
If you're gonna be late making a payment on your student loan whether Federal or non Federal then it's best to contact the Student Loan company to make arrangements with them before you do default.