Yes, anytime you can see daylight through the pores in the fabric......
They work GREAT for what I use them for: Pre-filters.
One on my wood chipper (Big 80 HP unit) before the oil bath air cleaner, another on my tractor when making hay.
I don't trust them for use as the ONLY filter - just too much dirt gets past. I had them on the turbo inlet on my Mustang drag car, and I was astounded how much dirt was getting past.