Defaulted student loans stay on your credit report for seven years (unless removed).
Defaulted student loans become collection accounts.
These collection accounts will lower your score up to 100 points. This will stop you from qualifying for credit cards, cars, and a home for at least three years.
The fastest way to remove defaulted student loans is by disputing. Learning to properly dispute can take time and requires studying the FCRA and FDCPA. Working with a professional is sometimes the fasest way to remove them. Otherwise they will stay for the entire seven year period.