There are no time limits to the Diffusion Cloud Free edition, or the Diffusion Free Trial.
The Diffusion Cloud Free edition is limited to 10/GB of free data per month. If you hit this limit, you'll need to wait until next month before being able to send more data. Alternatively you can upgrade to a paid plan, where there are no data limits.
The Diffusion Free Trial has no data limits, but should be used for evaluation purposes only.