it depends on the size of your nsx setup. You can easily upgrade NSX within a couple of hours with no downtime.
As mentioned, the amount of time will vary greatly depending on how many hosts and ESGs you have. The manager(s) will only take about ten minutes, the control cluster I'd guess would be about 30 minutes or so. Because you are already at a 6.3.x release, you won't need to reboot hosts as part of the VIB upgrade process, but the hosts will still each need to go into maintenance mode. This can take time if you've got a lot of VMs that need to be evacuated. All of these tasks can be done without any dataplane impact.
The only piece of the upgrade that is potentially impacting is the DLR/ESG upgrades. Having HA enabled will reduce the impact, but will not eliminate it. The ESG upgrade is essentially the same process as an ESG redeploy, where a new VM is deployed and then the old is deleted. You'll definitely want to anticipate momentary outages during this part of the upgrade.
I posted some upgrade tips on my blog a while back that may be helpful as well:
MikeMy blog: https://vswitchzero.com
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