I updated both of my TZTL12F units this past weekend. Elapsed time for the whole process was about an hour but I was doing some other things on the boat at the same time.
The update went smoothly and everything seems to be working fine. The Radar was still on the network and the sounder appears to be working OK too.
Just make 100% sure that you save all your points, routes, tracks, and settings as instructed before starting. The final step in the process resets the machine and wipes out all your data.
The only small issue I ran into was the Navpilot change that was mentioned in another thread. That's the issue where a "GoTo" command does not prompt you with the question about using the Navpilot to navigate to that point. To fix this go to settings and then routes and turn on "Navigate with AutoPilot".