15 mins is the shortest I would want to go. Otherwise, your app is going to be perceived as slow (by your users) because it has to do a bunch of needless DNS lookups a lot.
I doubt the perception in slowness is really going to be that different between a 5 min TTL and a 15 min TTL.
1. DNS lookups add on the order of 100ms to load time.
2. In both cases (5 min and 15 min) the user is going to do a DNS lookup on the first page, then have cached DNS while they browse for a bit, then have a DNS lookup at some point in the future.
Most web sessions are relatively short, so I doubt most users would even notice the difference where they see an extra 100ms load time at, say, 2 points in their session instead of 1.
In my experience the appropriate use of the preconnect and dns-prefetch hints has a much bigger impact on perceived performance than worrying about DNS TTLs beyond 5 min.
