Pinnacle does not indicate a fixed minimum volume.
From Pinnacle's fair use policy:
"The API is provided to players to facilitate wagering and may not be used for data gathering, scraping or for any other purpose. The API usage must be proportionate to wagering activities as determined on a case-by-case basis by Pinnacle." (my emphasis)
As I mentioned on a different thread, I maintained regular turnover, always used the "last" parameter when making calls for fixtures and odds (this is what poster "tango" refers to as the delta version), and only requested odds for the sports I was betting on. This is more or less what tango is explaining above.
I always used a 5 seconds refresh for odds (not for fixtures or other calls for which there is no need for such a fast rate - obviously people need to learn to use the API efficiently to minimize data consumption). My experience indicates that a monthly turnover (averaged over several years) of 250-300k is sufficient, but this is only based on my data. It's not too far from tango's estimate, though.