Good to know that it’s exactly 28 days. I thought it was 2-3 weeks.
I’ve had a lot of problems with that in the classroom, when SketchUp decides to log out in the middle of the lesson.
I usually log out and then in again at the start of a new course for 9 computers. The course lasts 2-3 weeks and I still occasionally see that I have to log in again when starting 3D Warehouse or LayOut.
What is more annoying is that when switching between SketchUp 2020 and 2019 I have to log in again and it is treated as another activation, so I have to log out of SketchUp 2020 first…
The classic licensing method was a lot easier for schools and ATC’s as the teacher didn’t have to think about logging in all the time.
And don’t get me started with what happens when someone decides log in to their 3D Warehouse profile during class - "Let me just log in to my profile using my Gmail account… oh it now requires 2 factor authentication… and now I need to go my e-mail account… what was my password again???.. let me reset it real fast… "
Actually, it may not be exactly 28 days. My suspicion is that it is this date next month, and so would be 28 days in February of non-leap years. Other times you may get a few days more. But, plan on 28 days and you should be safe.
There was a recent change in the Google two step thing. Now you can open Gmail mobile app and approve the sign in there. That’s a bit quicker than the email or texted code ways.
Recently, my internet connection was down for most of the day/night. When I tried to launch SU, I was required to login, which was not possible. I was unable to use SU. Is the 28 day clock running on my computer or the remote login server? If I stay logged in constantly, should my 28 day period never expire?
When you log in, a log file with the timestamp and what plan you have is stored on your local drive/computer.
SketchUp uses this to authorize the machine upon starting if it cannot reach the servers.
The max is 28 days (or the time that is left in your plan) That info is also stored on the serverside.
But it would need an actual log in to reset the counter, so best practice is to sign out after a session if you have issues with the internet.
Btw, it only takes a few bytes, so setting up a hotspot on your phone would only take a fee seconds to connect.
(Don’t start downloading stuff from the 3D Warehouse)
So, if I use SU daily and never log out/in, my 28 day clock is silently counting down in the background. Must have been an unlucky coincidence that the internet outage coincided with my 28 day expiration. Sort of feel like I am playing a game of “Russian Roulette” with SU and it won this time. Taking your advice of logging out/in on a daily basis adds one more unpleasant task to my workflow. Thanks for the clarification.
BTW, not everyone has a phone, or even one with hot spot service !
Taking your advice of logging out/in on a daily basis adds one more unpleasant task to my workflow. 
Thanks for the clarification.