How long can you go on a 50 amp extension cord?

I carry a 40 foot 50 amp cord as my main connection. I also carry 3 - 15 foot extension cords as I am sometimes a good distance from the power source. That's 85 feet with multiple junctions and I have run for more than a month with that setup. A direct 100 foot cable should perform even better. The cable uses 3 #6 and a #8 ground wire which I believe is pretty standard.