Waqar, I too have a green card with no expiry. Below is the reply I got from US Embassy Islamabad:
Hi,
I got my Green Card in 1989-90 at the age of 15 years. After that I took re-entry permit which also expired in 1991.
Since then, I have not travelled in USA and my green card is still with me.
I want to know if my green card has expired? can it be renewed?
Can I still move to US on permanent basis.
Reply:
Dear Sir/Madam,
This is to inform you that the maximum limit of stay outside of U.S. for a legal permanent resident (green card holder) as per U.S. immigration law, is less than one year (364 days). If a green cardholder has stayed more than six months and it is still less than one year, he/she can still travel to U.S before the stay exceeds one year.
I hope this information will be helpful to you.
U.S Embassy
Consular Section
Immigration Unit
Islamabad, Pakistan