You can build a home-made flamethrower by using a spray and a lighter:
(from this YouTube video)
But this can be dangerous if the fire gets in the can and the can explodes. So I guess the speed at which the particles come out of the box has to be high enough to prevent this.
NOTE: I don't want to do this at home, because I think it's too dangerous. This is just a thought experiment!
I guess the question "how fast can fire spread?" highly depends on the details (like: What kind of exothermic reaction is going on; how big is the pressure at which the spray exits the can; temperature around the can). I would like one theoretical answer (choose everything to make it as fast as possible / slow as possible) and a practical answer (make a reasonable assumption of what could go on with the lighter / spray stuff).