I'm all over the place, like hamburger.
It doesn't matter if I'm extra tired or not, sometimes I just rest and quietly think for an hour or so.
I'd guess the soonest I fall to sleep in about 15 minutes. I need 3 to 5 minutes just to get my body positioned in a comfy position sometimes.
I've fallen asleep on a couch maybe a dozen times in my life, never fallen asleep in chairs, on floors, etc. Not in front of the TV either.
Well, except one time, but that was more like passed out drunk. I woke some time later, and there was a horror movie on TV, and some girls head was being cut off.
That woke me up fast.