Expiration of cached pages stuck at 6 hours

Question

No matter what I change in the GUI, after clicking save the Expiration of cached pages under Performance options is reset to 6 hours. I checked my default VCL for Varnish suspecting this might be something set there, but couldn't find a 6 (hours) or value of 21600 (6 hours in seconds) there.

I'm pretty new to Varnish so not sure if it might be some default value, or what that could be called (or if Varnish is the culprit here at all, just a hunch).

We need to have Varnish keeping pages cached indefinately by default, and caches to only get cleared when content updates.