I am using this command with tshark:

tshark -r pcapfile "tcp and ip.src==192.168.1.1" -T text -V -x | grep 'Total Length'

This essentially parses the pcap for only connections from the source ip and looks for the total length in bytes from each packet. I get output like this:

Total Length: 125

Total Length: 210

Total Length: 40

Total Length: 125

etc, etc....

What I need to do is take the numbers from Total Length and add them up so I can get an idea of how much data was passed over the wire in the time frame of the pcap from a single IP. Is there a command I can add on the end of the one I am using to do this?

Write a script to loop while issuing that command, add a pipe using the cut function to remove the text for "Total Length:" and then add the numerical values together either by building an array or just adding iteratively to a variable. Each iteration of the loop you can output your total and then sleep for some period, say one second.

Not sure you can do it with just one command, although the whole loop can technically be a command typed at the command prompt, it's just easier to test, modify, and maintain if you make it a script. Plus you'll have to save the processed output to a variable as you add it up.