No Specific Rule
Generally it is better to apply before 90 days because,
incase if you dont get your EAD renewal before the
expiration of old one, you can go to local office and try to
get IEAD.
Note : I read a recent message about bcis holding applications
if you are applying too early that is before 120 days.
so it is better to apply when you have around 100 days left with the existing EAD