I haven't actually tried this, but editing /etc/crontab should work. Depending on your setup you could add lines for starting and stopping frostwire as so...
Code:
00 9 * * * root cd /opt/FrostWire;sh runFrost.sh
00 17 * * * root kill -9 `pgrep -f Frost`
You would then need to restart the cron service. This would start frostwire 9 am everday and stop at 5pm. Edit to fit your needs/setup (test on an actual command-line first).