That's a good question, which I ask myself. I found a lot of different information about whether I140 and I485 are being processed simultaneously or only one after the other. If in doubt, you should look at the Processing Times of the service center / field office responsible for your case and add up the times.
For example, in the case of my wife, the two petitions were received by USCIS on April 11, 2018. According to the Processing Times, the Texas Service Center is currently processing cases from October 27, 2017 (I140). And the Field Office in Philadelphia is currently working on cases from June 2017 (I485). This means that she has to expect to get the GC at the earliest in 16.5 months (5 1/2 + 11 months).