You can count time as a permanent resident since the date when you obtained the status of a permanent resident.
You can find this date in the Permanent Resident Card (green card). This information is located at the bottom of the card following the words “Resident Since:”.
At the same time, you should not be worried that this date may not coincide with the date on which you got a green card. Usually, it may be a prior date as there are situations when a green card is sent to the actual US address of the immigrant, so you will see the actual date of its granting.
However, if a green card is not with you at the moment, there is one more document that will indicate the time as a permanent resident. Look through the I-797 Notice of Action or consular letter that serve as a grant of the permanent resident status. The date indicated in the letter is actually the one when you got permanent residence.
Also read about Form N-400.