Just a gut feeling. 7 kind of felt too short, and 14-30 a bit too long. We feel that you'll experience the benefit in 10 days. We can always adjust if needs be.
Startups are an experiment after all, so let's see what works.
Currently thinking about having a 7 day trial that starts ticking after say 10 successful API calls. Do you think that is a good compromise?
Have you thought about N API calls for a trial? That's what I've used for API-first products in the past that worked pretty well for me.