Same oscilloscope trace with cursor measurement of time interval between the instant at which the surge voltage rises to 0,5 of its peak value, and then falls to 0,5 of its peak value. Duration is 18.00 μs.
https://cybre.ams3.digitaloceanspaces.com/media_attachments/files/108/719/790/052/332/815/original/d759dada4544a1ea.png