Remote NGFW upgrade should take few minutes in most environments. How upgrade works is that NGFW engine has two partitions for the software. The active partition holds the currently running version and inactive partition holds the previous version. When you launch remote upgrade, SMC will first upload the new software version to inactive partition on the engine so first thing that effects the time upgrade takes is how fast is the connection between SMC and NGFW engine and how long it takes to upload the new image to engine. Once image is uploaded, engine does quick check that every looks good, reboots, and boots up to the partition where new version was installed. Reboot process usually takes from 2 to 4 minutes depending on the appliance model.
To clarify further, if you have a single firewall then the network is down between the time firewall shuts down for reboot and has booted up completely. So 2 to 4 minutes usually as Tero said. In a cluster you can upgrade one node at a time, and there will not be downtime.