How to schedule tcpdump to run for a specific period of time?

You can combine -G {sec} (rotate dump files every x seconds) and -W {count} (limit # of dump files) to get what you want:

tcpdump -G 15 -W 1 -w myfile -i eth0 'port 8080'

would run for 15 seconds and then stop. Turn 1.5 hours into seconds and it should work.