It varies. It took effort to light it with a Bunsen burner to demonstrate synthesis reactions when I taught science. I mixed it on a small square of paper and set it on a mesh screen over the burner. I seldom had the perfect mix, which I recall was 6 to 1 by weight, but after the first time weighing it out, I always just estimated the size of each pile. It lit after the flame got on it a few seconds and provided quite the show, stinking up our whole wing. The other teachers hated it, which delighted the kids and myself.

The more fine the powder, the easier and quicker it went up. I used flowers of sulfur and I don't recall how fine the zinc was, but I didn't try to mill it down any finer. IIRC, it can be friction, static and shock sensitive to some degree depending on how it is prepared, particle size, the phase of Saturn and Uranus, how you hold your tongue, and whether or not your cat has recently puked up a hair ball.

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I love sanding.