You can immediately see the change in length in the info line, so marking the length before and after you will have the difference.
Furthermore you can open the time stretch process and enter the resulting length taken from the info line and compare it to the original length (as it always shows that info). That will tell you the change in samples, BPM, and stretch ratio. Specifically for BPM you can input a different tempo than 120 at the original field (if you know the value) and will always give you the resulted BPM.
Hope that helps, cheers
*EDIT:
Wait I just realized that the video is not showing the popup window for the time stretch process so I am uploading a ss.
