I did 90 seconds today. I'm finding that the rope length means that I am too close to the floor. When I ease down, I partially rest my head on the ground and raise it up off the ground a few inches to take the full strain.
This means that my head is almost horizontal to the floor and this position is very hard to sustain. The answer is simply to raise the rope several inches more, so that I can let my head drop and my body can take less strain.
I have also tied another rope to the rafter so that I can pull myself up with it, rather than straining up to grab the suspension rope.
I'll try for 2 minutes the next time, if it's a good day!
Your pal, TIM.