Too long by what metric? Personally, I dislike short trials. If I get distracted by something more important before I have time to fully investigate the product, I'm likely to just forget about it when the trial prematurely ends.
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.
Startups are an experiment after all, so let's see what works.