To clarify, EAD is an interim work benefit that allows you to work while your I-485 is pending. You are eligible to apply for this benefit anytime while you have a pending I-485 application. You should receive the EAD around 3 months after you've applied for it, unless your I-485 has already been adjudicated. The the moment the I-485 is (hopefully) approved (which may include waiting for pending name checks etc.), your new employment authorization is then based on having become a permanent resident (your proof of employment authorization becomes your GC, I-551 stamp, or SS card without a DHS restriction). I hope this helps!