I would guess (underline guess) that an 8 month to a year delay from start of deployment in a given area until it is operational.  It would vary depending on the area of coverage of course. VZ and T are beginning deployments in selected areas this quarter, so I would guess sometime after September 2019 is when these networks are in full use by customers using 5G handsets.

Re handsets, I believe I heard that Samsung would have their 5G handsets ready in the first half of 2019, whereas Apple would sometime in the second half of 2019. September would be my guess, since that is when Apple traditionally releases new models.