Assuming nothing goes wrong, the downtime should only be a few minutes during reboot. Also note that you can use freebsd-update which does a binary upgrade, rather than compiling from source, though it will still require a reboot.
There are always potential problems--such as a library change breaking a 3rd party program, but usually such problems are mentioned in UPDATING.
You can compile the sources for world and kernel while the system is running, so there's no downtime there. Once you've performed a few updates, I would bet you could install the system and use mergemaster(8) to merge your configuration files within a minute or two. Then it's just a matter of rebooting if the kernel was updated.