The screen type spark arrestor need over 9 sq in of new screen to have equal flow area to a 1.25" ID tube.  The screen type of spark arrestor needs a lot more screen than 9 sq in to over come the flow restriction created when the flow is split up as it flows through more  than 4000 holes in 9 sq in.  A lot more screen must be added to compensate for the carbon that forms on the screen mesh and to get equal flow resistance to a non spark arrestor muffler.

Trying to determine how quickly the flow deteriorates due to carbon is difficult because of the different riding styles and the oils and oil mixtures used. 

My estimate of the amount of forest approved screen would need to be over 40 sq in.  I think that 40 sq. in. or more of screen would allow similar flow and maybe allow at least 10 gallons of fuel to be run through the engine between screen cleanings.