My Garmin updates at random times, but usually when I do a ramp test. It is always a couple percentage beow the ramp test number, but close enough that training with either number I think would work fine. Last time I had a Garmin FTP 3 watts below the Intervals.icu estimate, which was about 3 watts below my ramp test a week before.
I did have an update pop up after a hard workout last week that put my 3 watts over the ramp number, but I am scheduled for a test next week.