Actually you can theoretically go indefinitely if you have a 'sleeping facility' available and you can 'have a rest' there. Because 'sleeping time is not calculated as duty time'. There is wide determination as to what a 'sleeping facility' is. My company has argued that the flight deck, while on delay, constitutes a sleeping facility. Also undefined is when the 'sleep time' begins or ends. My company routinely schedules days well in excess of 16 hours, with only two pilots, but has a built in 'sleep time' of five or six hours in the middle. Of course that sleep time is routinely reduced or eliminated due to delays, which leads the company arguing that if we rested in the flight deck during a delay then we are legal.

I was threatened with suspension once last year if I refused to accept their defining the flight deck as a sleeping area. I stood my ground and they backed down, but I did get accosted by the replacement pilot who was called off of reserve. Recently the company has not been backing down though and guys have been fined and/or suspended for refusing to fly.