It depends on the type of student loan but you have to wait a minimum of 20 years before your student loans are able to be forgiven and for some student loans you have to wait up to 25 years before the student loans can be forgiven.
You do however have to be enrolled in a payment plan for the student loans during those 20 to 25 years and be making payments towards your student loans for your student loans to be eligible to be forgiven.
Do know that not everyone is eligible for student loan forgiveness so you may have to continue to pay your student loans even after 20 to 25 years until the student loans are paid off.