Yes, set "Accounting" in addition to "Log" in the Track column of the relevant rule; this will show the egress interface as well as time connected, payload bytes sent/received etc. Accounting updates are added to the original log entry every 10 minutes or whenever the connection ends by default. Note that this will increase the logging load generated by the gateway and sent to the Log Server.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones