Honestly, we used to replace a lot of pam_ldap and similar sorts of awful solutions. With those, if your LDAP went down even for a heartbeat, you couldn't log in at all.

So I totally agree: if I had to do certificates and didn't have something like Userify, a 1 hour (or even shorter if possible) expiration seems quite worth chasing, especially with suitable highly available configuration. (Of course, TFA doesn't even bother mentioning revocation and expiration, which should give you a clue as to how much fun those are lol)

And for more normal, lower-security requirements or non-HA, 6 or 8 hours or so would probably work and give you plenty of time for even serious system outages before the certs expired.

Not to hard shill or anything (apologies in advance, just skip if you're not interested), but there are two significant security and reliability differences between standard SSH (with or without certificates) and Userify:

1. Userify Cloud updates by default every three minutes, and on-premise Userify Express/Enterprise updates every ten seconds, but it doesn't have to update at all; even if your Userify server goes offline forever, you can still log in because the accounts are standard UNIX accounts (literally created with `useradd`)

2. When accounts are removed, Userify also completely nukes the user account, removes its sudo perms, and totally kill -9 's any tmux/screen/etc sessions (all processes owned by the user are terminated across the entire enterprise within seconds), which is also not something that a certificate expiration would ever do.