A data viewer report will take an indeterminate time to complete - depending on the complexity of the report. For that reason, when called from the API it is called using two API endpoints,

Start - this starts the process
see Start

Check status - this checks the process, and returns the data if it is ready
see Check status

Note: There is also an option in the Start endpoint to pass a "wait" parameter. If this is passed the system simply waits for the period selected and then outputs the data - saving the user having to make the check status call.