I figure it by years more than anything else. They are cheap and easy to change and fresh ones always make a noticeable difference to me. Seals are much better than they once were, but I am still very reluctant to run past 24 months and more often than not 12 unless I get clues that they are bad sooner. My favorite test for a mag side seal is to start it up, get it warm, take off the mag cover while holding it at a constant high idle: If it picks up speed and sometimes it will really start to sing, you know it just went lean as the cover came off, and you now know the seal is REALLY suspect. If you have a really good and clear vent like some people are prone to run, this test will tell you nothing. Sometimes a rig like a honda which has a clutch cable entering the magneto cavity you can wiggle the cable, lift up a bit etc and get the idle to change. Turning the bars will do it too on occasion.
You ask about cylinder lube, and I prefer nearly dry or even dry to lubed. At the urging of a friend of mine who has built multiple national winning motorcycles, I do my final clean with atf and then rub the cylinder with cloth or paper towels until it comes up clean and is as dry as the towel will leave it. I wet the bearings all the way around, dump a bit (1/2 oz to 1 oz) into the crankcase, lay the bike over on one side for about 5 mins, then flip it over. that lubes the mains.
