My daughter and I tried it out. A 1/4” sliver off the mushroom she has in the picture lasted about 1 mile or 15 min, so 3 mushrooms will move a spark for about a day; however it catches a spark so easily, I would just star new at each camp sight. It does not work if the mushroom is wet or too moist, dead black rotting ones work best. It also would be a great hand warmer on winter.
It’s freezing again so I did an experiment. First, I lit a dried out mushroom with flint and steel. It literally took the first spark that hit it. Second, I used a small mushroom about 2 inches as a hand warmer. It was amazing. I will do it again and take some pictures, but it lasted about 2 hours and I could put it down in the snow do some work and com back to it without a problem.