[toumhi]

A free trial should help a potential customer decide whether or not the product is a right fit for his need.

So if you offer a slow, limited version is it helping your potential customer to do so? No, by making things difficult you're just manufacturing frustration.

From the other comments, it seems visitors need only a couple of hours to evaluate it. So why not make it a free trial available only for 1 day or for 3 uses?

Your fear("my worry is that somebody will get their money's worth in 14 days and choose not to buy afterwards) implies to me that you're not sure how your product fits in the user day-to-day workflow. If users would use it for 14 days and leave, the problem is not in the free trial, it is in failing to provide value and engage users.

So how can you change your product/free trial to engage users, getting them to realize value of your product, and creating habits for them to come back to your product?

I wrote an article recently into designing free trials, it might be of interest to you: http://www.saasfoundry.io/blog/design-saas-free-trial/

Good luck!