You should request a new green card every 10 years providing you are a lawful permanent resident. To renew your green card, send a completed Form I-90 to United States Citizenship and Immigration Services six months before your current card expires. You will retain the permanent resident status even if your green card expired. However, you should immediately apply for a new green card if the current card is no longer valid.
If you are a conditional permanent resident, your green card is valid for 2 years instead of 10. Instead of using Form I-90 once your green card has expired, apply Form I-751 or Form I-829 to remove the conditions to residence. Conditional permanent residents cannot renew their green cards if their cards reach the expiration date.