CORRECTION: If the marriage is more than two years when you are ISSUED a green card, It IS a permanent.
This means, do not apply green care right after marriage, wait 1 year, it ussually takes about 1 year to get approved after you submit application. If your timing is right, the shortest time to get permanant green card is two year, you are issued the card right after your 2-year anualversary. The longest is up to 4 years, in case you got your green card right before your 2 year-anualversary.