@alison-rafaelc Hi Alison,
It depends on how you architect the FusionAuth system, but in our FusionAuth cloud system, we see downtime of seconds to minutes for system upgrades (depending on the number of users and the data being migrated). We get this by swapping out one node at a time, running in an 3+ node cluster. Some details here: https://fusionauth.io/docs/v1/tech/installation-guide/cloud#upgrade-duration
Worth noting that you control when you do the upgrade, unlike a typical multi-tenant saas.
We have a number of customers and community members with thousands of tenants and millions of users so the numbers you mention seem fine. I'm glad you were able to stand up a FusionAuth cluster and load test it.
Zero downtime upgrades are something we've discussed internally and have a strategy for, it just hasn't made it onto the roadmap yet.
If you'd like to have a discussion with a technical sales team about FusionAuth and SLAs, I'm happy to have someone reach out to you.