By definition only a subpart of your fulltime users will convert to paying customers. So in order to be able to try the pricing I would say you must have at least 20+ fulltime customers and you should also be able to generate like 10+ fulltime customers a month. If you have less than that I would say it's better to focus on making the product better and increase conversions from signup&disappear to fulltime users. Once you have thise basic numbers it would good and brave to try the pricing.

1) Ask a couple of customers what is fair 2) See what competitors are offering

then just publish a pricing. You will learn a great deal and iterate.