I saw this question in the archive and was also looking to find the rtp ptime by looking at the RTP and not using the value in the SDP to confirm what it actually was. I was searching through wireshark and found there was a graph that could be displayed for the RTP which shows packet intervals but you can use it to get the packetization time assuming there is no jitter (or if jitter is very low)
Path for my version (1.8.15) has the path as Telephony/RTP/Stream Analysis and in that window that pops up there is a button that says graph.