A friend of mine had a bag of some apparently high-potency weed that you could smell from a mile away while still sealed in a bag. He had the remnant of the bag several weeks later and it was practically odorless compared to before. He said he thinks it’s less potent now, but can’t be sure due to fluctuations in usage, his tolerance, etc.
I wouldn’t think that THC evaporates, but the reduction in odor after a few weeks, and people’s tendency to label the really pungent stuff as “good”, makes me wonder if older weed has less kick.
In my nonscientific experience, not having subjected most of my chronic to a GC/MS:
Pungency is often related to the moisture content of the herb than anything else - there seem to be some fairly volatile compounds that evaporate off an unsealed bag of weed pretty quickly - on a scale of weeks, I’d be skeptical that you’d really notice a difference. Super-dank odorous stuff is usually just wet, in my experience.
I can, however, verify that marijuana left for several years will indeed oxidize a deep red/brown and become almost entirely unenjoyable.
I read once (in High Times, I think, but can’t be sure) that pot begins to lose potency noticeably after a year, and by two years is practically useless. When I smoked, I never had it long enough to test it, though.
I know a guy who keeps a ‘special reserve’ of nice buds that he’s come across, some of them (he claims) have been in there over 3 years. On special occasions he’ll pull out a bud, pore over it for a while, take a big sniff and proclaim “Fine piece of Kali Mist there. 2009 vintage, if I’m not mistaken”, before crumbling it into a joint to share round. Never gets any complaints!
Apparently it keeps best if it’s in an airtight container, packed full so there is as little air as possible in there, and it must be completely bone dry before sealing, or else it will sweat and go mouldy.
He claims not to have seen a noticeable drop in potency (but after 3 years, how can you be sure, it’s pretty subjective after all) although it loses some of it’s flavour after a year or so, but there is always a pungent smell when the jar gets cracked open.