Two really good dogs only about 5 to 10 minutes at the most. Two "just dogs" never.
It all depends on the dog. My yella dog can kill a coyote single handed in less than 10 minutes almost always. Its not a fight though at all, its just a matter of blood flow and oxygen depletion. Any dog that "fights" a coyote to death will quit. Maybe not now, maybe not later but soon enough to make you realize just how shitty he or she is.
Two really hard core dogs like John and Stitch on a coyote, it would be dead within five minutes I'd say if they were young, healthy and fit but they are