It varies wildly. Some people are very lucky and get their Green Card within a couple months of filing and find EAD to be useless because they're approved before it ever arrives.
Likewise, there have been many, many people who have been stuck in name checks for months, even years, and would probably be going insane without their EAD. Even those who aren't stuck in name checks (perhaps were transferred to CSC and are waiting there

) find that their EAD is like gold.
Basically the EAD and the $180 you pay for it is like fire insurance. You hope and pray that you won't need it and will end up being useless, but if you do need it, you have it and you're very glad.
In summary (because I get very rambly in the evening): there is no way to tell when you will get your Green Card. If you're concerned about working, I'd apply for EAD immediately. You'll almost definitely have it in 90 days.