If you enable scheduler you see 4 options
Start Time
End Time
Duration
Startup Delay
As per official JMeter guidelines, you should not use all of these 4 together. In your case, I will suggest you to either define start & end time or just define duration.
For details refer to ..
https://jmeter.apache.org/api/org/apache/jmeter/threads/ThreadGroup.html
Version 1.9 introduces a test run scheduler. Click the checkbox at the bottom of the Thread Group panel to reveal extra fields in which you can enter the start and end times of the run. When the test is started, JMeter will wait if necessary until the start-time has been reached. At the end of each cycle, JMeter checks if the end-time has been reached, and if so, the run is stopped, otherwise the test is allowed to continue until the iteration limit is reached. Alternatively, one can use the relative delay and duration fields. Note that delay overrides start-time, and duration over-rides end-time.
http://jmeter.apache.org/usermanual/test_plan.html#thread_group