How to change a meta value (of a published post) after X days.?

Question

I have a custom field. I would like to change the text of that custom field after x days.

I want something like this:

if ( published post is more than 90 days old ) {
    echo "your post is more than 90 days old"; 
}
else {
    echo "your post is less than 90 days old";
}

Can anyone help me to achieve this?

Thanks.