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 Gateway Performance Optimization R81.20 course
CET (Europe) Timezone Course Scheduled for July 1-2