How can I track my subject's filling time?

e.g how long it took each subject to fill the form?

Hi,

You may want to check on analytics to better understand how your users fill out the form. You can learn more about that in this guide: https://www.jotform.com/help/425-Understanding-Form-Analytics-and-Ways-on-How-to-Get-Your-User-s-Info. 

I hope that helps.