I'd find the number intersting how many users would pay, if there were no free option available - my take is that it would be much more than the one percent of users that seem to be the rule in the freemium model. Of course a free plan attracts more potential users in the first place, which means the cake is smaller, so it's hard to evaluate what yields more paying users...

The more important factor to me, though, is cost and that usually not only involves storing some data as in the case of historious, but also support requests and scaling costs - when you have a paying user base scaling just won't be that much of pain point.

So, I'll go with a free 30 Day trial period after which the user has to decide, whether he'd like to sign up and pay. That way I can easily handle a couple of thousand users on one server and have more time left to spend on development instead of suporting nonpaying customers.

Free or Freemium only makes sense to me, if the presence of more users provides value to paying users, e.g. okcupid, Skype, and other network-effect businesses.