So you need to run that command
Add two entries in your cron !
0,10,20,30,40,50 * * * * route add 0.0.0.0 mask 0.0.0.0 192.168.1.2 >> /dev/null 2>&1
Another one :
5, 15, 25, 35, 45, 55 * * * * route add 0.0.0.0 mask 0.0.0.0 192.168.1.3 >> /dev/null 2>&1
That way.. it will change gateway every five minutes.