Ask HN: What's the best duration for a free trial for a PaaS?

Hi I'm working on a PaaS project and I need to have a built-in trial. The most common trial period I've seen is 14 days. What's so magic about this number? Is it the two full weeks? But why? Anyway, I want to squeeze the trial period a bit down and optimize for a better conversion rate. Also, a shorter trial period saves me cash hardware-wise as I'm bootstrapping this project. This PaaS requires one to change DNS. So I need to account for DNS changes ("propagation"). So if I allocate 2 full days for DNS and one weekend, all amounts to 9 days. Is 9 days too short? Thanks!