How do I know when preview will work offline?

The C3 editor works great offline. Previewing projects seems to work for some time but eventually stops loading. At first I thought it was a time issue, like it will work for say 2 hours but needs to connect to the server again. Now I suspect it is related to adding new types of objects or something like that. I haven't been able to do controlled tests though because it takes so long to fail. I have some travel coming up and would like to keep working while in transit.

Are there a set of rules for when previewing projects will stop working offline?

Construct will keep working offline so long as the browser keeps its data saved. It's not in Construct's control. Perhaps check your browser settings - if you have privacy settings or browser extensions that do something like clear storage (possibly referred to as "cookies", a term I think is a bit misleading) periodically, then that will possibly also clear the saved data for Construct to work offline.