In Microsoft Flow there are different Plans available and for the free testing there is Microsoft Flow Free available. This plan is available for everyone with a live id or a work/school account.
The Plan Free does not have a time limitation f.e. For 30 days. It is a free version with no end date. It is “only” limited in the usage and functions. The free version allows 750 runs per Month with a maximum flow frequency of 15 minutes. Other limitation are f.e. In sharing and collaboration which is both not available in the Free plan. The possibility to use data gateways to connect to on premise tools is not given in the free plan also the common data service as an premium service is missing. The free plan has only the standard services and not the premium services available. For the Free plan is no service level agreement available.
With the licenses of Office 365 and Dynamics 365 there comes the Plans Flow for Office 365 and Dynamics 365. These are paid plans but they are paid by the license for office or dynamics. These plans allow 2.000 runs per month per User with a maximum flow frequency of 5 minutes.
With the office/dynamics plans the connectivity to on premise data is allowed using the on-premise data gateway. The created flow can be shared with other user in the same organization.
The plans Flow 1 and Flow 2 can be purchased as a separat license. Flow 1 for 5 $ and Flow 2 for 15 $ per user per month. Flow 1 allows 4500 runs per month per user and Flow 2 allows 15.000 per month per user. The maximum flow frequency is for Flow 1 3 minutes and for Flow 2 1 minute. With these two plans the pr3mium services f.e. Common Data Service or Salesforce can be used within a flow. The on-premise data gateway can be used to connect to on-premise data.
Here you can find the actual pricing for flow.