Instant action matches always start with the ending conditions of 10 frags or 5 minutes, but how can I change that default to something longer?

You can't, and that's the very point of instant action

Timelimits and fraglimits are fetched from .instantaction files, in the map directories from the data pk3.
They set fraglimit_override and timelimit_override, so it can't be changed.

Ookey, thanks.

My pleasure