Two answers being FIRST that one must keep their visor CLEAN & NO SCRATCHES.
Second would be by keeping the visor above cleaned constantly. One can use Plexus, SlipStreamer, most plastic protectant materials like Armor-all, 303, & such to even Pledge.
I do this each time I come home from a ride & ALWAYS use a clean cotten piece of cotten (often one of my many retired T-shirts). On the hwy I will clean the above again at half-way point to even more often if I am bing splattered by bugs FOR I carry one of the cleaning liquids & rags under my seat. DO NOT use the same rag time & time again for they are so easy to wash by hand & AVOID the normal paper rags as they will scratch the plastic visor. I have something like 10 or 12 cotten rags around all the time.
So you are going to say "--but this was rain----" & I will agree that rain is a bummer, STILL with a clean visor & with the speed up a bit just turning the head left or right & most of the rain (which is also road crap thowen by cages) will run off WITH a well cleaned visor for it will be slippery.
At home I use Pledge & that also allows me to put more of it on the helmet itself & they look 100% new with no scratches.
Said visors can last a long time if the above is done. For instance I have a 1 yr old & believe it is a 3 - 4 yr old Shoei only fitted with visors that are around 6 yrs old while the new ones are stored away for future use.