How and when calculated field "Time Spent" in the request

How and when calculated field "Time Spent" in the request

Hi.
 
Could you please advise how and when calculated field "Time Spent" in the request. For some our requests this field is filled with some strange time, for others this field is 0h00m.
I've tried to repeat but don't get the algorithm for the Time Spent calculation.
Thank you in advance.
 
Regards,
Andrey

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