Calculating range between 2 points within higher grey value

Dear ImageJ and FIJI community,

I am currently trying to calculate the duration when higher gray value is detected as shown in the Plot 1 Image marked by red square, and expected duration by blue line.

Plot 1 marked.tif (202.6 KB)

My question is:
Is it possible to calculate the duration using ImageJ ? If so can I have a guidance to do so.

Here I also attached the Plot Values of the image.
Plot 1.rar (19.6 KB)

Many Thanks in Advance for your help.
Best Regards,