They are limited as far as distance goes, by fuel capacity, weather conditions, etc.
I think 40 miles south of Montauk, you stand a good chance of one of them coming out, provided, the weather isn't dangerous, in which case, Coast Guard would be your option.
At the Canyon, you won't see Boat US or Seatow. Not happeneing, it's too far, and they don't have the fuel capacity to get out there, hook on, and tow you back for 8 hours.
Best bet is to talk to your local tow companies, they all differ in their operation, some are like night and day.
Also, you rely heavily on fellow boaters out there. I towed a guy in from 27 miles out, thru the worst weather I have encountered out there. It blew up out of nowhere, from the Northeast, about 35-45 MPH winds, with downpouring rain, and thunder, lightning all around. It was just the guy's luck that his brand new "black" motor crapped out on him. I saw his flare right before I headed in. He was lucky.
It was a sloppy and miserable tow, but there certainly was no alternative
I would want the same type of assistance if it were me, so I am always more than helpful. Last year I towed a guy that was drifting just south of Narragansett Bay also with a dead motor.
We're "all in the same boat" (I know, that was BAAAD), so we need to help each other.