We don't yet have a committed time we'll keep you warm and it might change (<-- this is me hedges against future change :)), but it's about 5 minutes today. That's ANY function within a collection of functions running.
For background: I maintain OneBusAway for iOS, and am investigating options for sidecar data (e.g. how delayed is a bus, is there still room on it, etc.). Ideally, this would all be part of the core OBA API, but that may not be an option. I'd just use Apple's CloudKit, but I want to be able to process incoming data and share the data between iOS and Android. Hence my interest in your product :)
FYI - You can reserve a VM, if you want, but then you're paying until you turn it off. It will always stay hot, though.