Yes, you will hit the limitations of the free plan very quickly indeed, at which point you should start paying.
The idea of these free plans is to let tire-kickers try out the system. They don't have to pay to test drive the UX and deployment mechanisms, which are the value-added features that sell a platform like Appfog. They don't have to pay for low-traffic prototypes. They can launch a whole set of minimum-viable sites on free hardware and then upscale one within a few minutes of any traffic spike (in theory, anyway). And it empowers marketing maneuvers like getting an entire programming class of 25 people using your platform for a one-day class.
(I'm not sure I love the above business plan, to say the least, but that's the idea, and thanks to Heroku it has become the standard.)