A user does not submit requests continuously. A user submits a request, the server receives and processes the request, and then returns a result, at which point the user spends some time before submitting a new request. The time between one request and the next is called think time.
Think times are dependent on the type of users. For example, machine-to-machine interaction such as for a web service typically has a lower think time than that of a human user. You may have to consider a mix of machine and human interactions to estimate think time.
Determining the average think time is important. You can use this duration to calculate the number of requests that need to be completed per minute, as well as the number of concurrent users the system can support.