It depends on the size of the roast and weather conditions... Mainly wind.
Smaller ones have taken 2-2 1/2 hours average. (7-10 lbers)
Larger ones... (18-24 lbers) usually take longer (4-5 hrs.) but...
I've figured out that by encasing them in rock salt, you can really crank up the heat and still end up with a tender juicy result.
I also use a cable probe thermometer, mainly because of the cost of the larger cuts... So how hard can that be?

I usually dig the roast out of the salt when the meter hits 135, then tent it with foil for 15 minutes or so, with the temperature usually finishing off at 145-150.
By laying the heat to 'em, I've found that the big monsters can be ready in as little as 3 hrs.


Ready for burial...

burried...

this isn't really "layin' the heat to it... would be two solid rings of briquettes...

mmm-mmm-good...