On the question of how often a backwashing sediment filter should be backwashed, there is no concrete answer. The common way is simply to guess. If you're guessing, err on the side of too much rather than too little, because excessive sediment buildup can sometimes be irreversible.
The best way to make the when-to-backwash decision is to install pressure gauges before and after the filter. Be sure to backwash before a critical pressure drop is reached.
According to Joseph Harrison (Water Technology, Nov. 2005): "At pressure drop increases of 15 or more psid, the captured suspended solids can begin to form a more compacted filter cake, which may become difficult to break up and remove during backwashing."
As for service flow rates for sediment filters, keep in mind that the best efficiency is at lower flow volumes, and keep in mind especially that recommended flow rates are normally stated in terms of gallons per minute, per square feet of filter surface. That's square feet, not cubic feet. A 10" diameter filter tank may hold 1.5 cubic feet of media, but its surface area in square feet is only about 0.55, a bit over half a square foot. Consider another statement by Harrison:
"Multimedia beds are sometimes effectively operated at flow velocities as high as 20 gpm/ft2, but their best efficiency is perhaps when they are operated in the range of 3 to 5 gpm/ft2" In terms of your 10" X 54" filter, this means that you might push it to an 11 gpm flow, but you'll get best results at 1.65 to 2.75 gpm." (Water Technology, Nov. 2005)
Yes, nothing lasts forever. Five to ten years is a good guess at the lifespan of sediment filter media. A certain amount may be lost to attrition. As it degrades with prolonged service, some of the media will be washed out during backwashing. What stays in the tank eventually loses its effectiveness as the irregularities of its surface are eroded. Also, there can be particle buildup that isn't corrected by backwashing which will lead to significant pressure drop.
(whole house & well units)