First off, I have reloaded over 1 million rounds of ammo over the years (I had a commercial operation in Tucson way back when and used 2 Dillon 1000 presses, one for large and one for small primers). Mostly I used WW primers because they were easy to turn over into the tray to load my primer tubes. Sometimes Federal, if there were no WW to had.
I looked closely at anyone that brought in a defective round, which didn't happen very often. It always turned out to be something wrong with the gun, probably 99% because someone had done their own trigger work or replaced the springs with lighter ones. I was never once able to say with certainty that it was a bad primer.
I also did some testing with primers, exposing them to various things I had laying around. I tested after 8 & 24 hours, with these results. All cases were primed with WWLP primers on the Dillon press in 2, 45 ACP cases and left upright for the duration. N = normal.
S = squib 0 = nada!
8 hrs. 24 hrs.
Water, 2 drops N, 2 N, 2
Rem oil. 2 drop S, 2 S, 2
Kroil, 2 drops S, 2 S, 2
Alcohol, 2 drops. 1 lite, 1 N. N,
Parts Cleaner 'spray' as above
WD-40, 2 drops 0, 2. S,
The water really surprised me when it didn't seem to affect them at all. Clearly WD-40 is a primer killer but when left for 24 hrs. it had evaporated enough to at least get a squib. I'm not sure if the squibs would have set a charge off or maybe caused a hang fire. The alcohol lite would have definitely set the charge off but no telling about accuracy. I also carried 10 primers around in my pocket for 10 days (WWLP) and all of them worked just fine. They were handled every time I got my keys out, etc.
The bottom line is, unless you have really sweaty hands, get them wet for a while or douse them with WD, they are very likely to go off. Casual handling of primers (e.g., picking a dropped one on the bench or floor) with your bare (non-greasy/sweaty) hands/fingers will NOT kill your primers! These things are tougher than most realize. In fact, I had some of my shop .45 ammo (200SWC, 5.5/231) go thru the wash (were they ever clean!) and they worked just fine a week later. However, this not to say they can't be killed!
I have some 7.62 x 39 WW factory ammo that a buddy left sitting out in his basement for 2 - 3 years, some FMJ & some SP. The boxes are perfect, no water damage at all. Yet both of them have occasional misfires with deep indents in the primers (SKS), which did not fire at all. I pulled the bullets from those and found the base of the bullets corroded and the powder clumped together. These rounds apparently sucked enough moisture out of the damp MO air to kill them. I sorted them by shaking the cases. If I could feel the powder, they were good. If not, they got pulled. Most were ok, the SP's were loaded with a fine powder I couldn't really feel. I just resized the case neck & reseated the bullets. I tried putting some known bad rounds in dessicant but even after a month it was no help. Oh well.
I suspect that most failures to fire are a fault of the shooter or reloader. I've been guilty of not seating them deep enough and having the primer cake break up from the first firing pin hit and of course, not go off at all. You can tell because the interior if the case will be the color of the now-powdered primer compound when the bullet is pulled.
Time wise, if kept dry primers will essentially last forever. I've fired primed brass from 1917 (!) that worked just fine. Same with ammo loaded during WW-1! Keep them dry & free of the humidity, free of things like penetrating oil and they will do their job.
Cheers,
crkckr