The principal reason for the screen is to capture large non-magnetic particles such as aluminum shavings and adhesives that are present after initial machining and assembly. You should expect to see and remove this debris at the first one or two inspections.
Subsequent inspections will still offer up some/few particles, but there is little concern that their presence on the screen will restrict oil flow, so the need for inspecting the screen diminishes with time.
There are some early bikes (1989–2002) that experienced problems with the aluminum oil gallery plug backing-out causing a lot of aluminum shavings to be deposited on the screen, so if you have one of these bikes I suggest inspecting more frequently. It will save your engine if you see this symptom.
Early bikes also experienced problems with rocker arm chrome (also non-magnetic) flaking and being deposited on the oil screen so inspecting the screen for these bike yields useful information. (see photo below)
Here’s how you tell the difference between chrome and aluminum particles:
Battery acid is dilute sulfuric acid H2SO4
Chromium is soluble in dilute sulfuric acid
chromium + sulfuric acid yields chromium sulfate + hydrogen gas
2Cr + 3H2SO4 > Cr2 [SO4]3 + 3H2
So, all that you need to do is to put the flakes caught by your oil screen into a small amount of acid drawn from a lead-acid battery. If you see hydrogen gas being rapidly given-off and the color of the flakes darken to the violet hue of chromium sulfate, you’ve probably got flaking rockers.