Usually when I update it takes no more than 5 minutes max, although I've never timed the reboot process. Note, it's going to depend on your hardware. I'd say, though, if it's been a decent amount of time, and you haven't gotten any sign of it responding (after pinging it, checking the systems' indicator lights, etc.), I'd probably plug a monitor into it and reboot the machine.

Over the years I've also had my fair share of updates go wrong on TrueNAS, especially with Core. I suspect that it had much to do with the worse hardware compatibility of FreeBSD which kept introducing problems for me. Things have been a lot smoother since I've been using Scale on both my machines.