Awhile back I got curious, so I checked my customer profile on Adorama (they keep a record of everything you've ever ordered). Virtually every lens I've purchased is either now selling for more or roughly the same as what I paid.
So, I guess the answer would be: maybe never.
Untrue to one extent: Crap always varies. Good stuff on the other hand is on a "buy it if you need it" basis . . . because it's a hit-or-miss sale proposition.
The only lens I got (financially) burned on was the 70-300 IS . . . which is also the lens I was greatly disappointed in.
As I said in another post (paraphrased) it's not like X bursts into flames and stops taking good photos when it's replacement comes out.
Most people are looking for sales on the 24-70mm right now; so, just find a 'good' dip in price ASAP and stick with it, IF you need it and you'll be fine.
I don't know about that, most glass prices seem to remain fairly consistent. Hell the original 24-70 was $950 when it came out! And it was as high as $1449 retail within the last few years and it's still $1200 at B&H. Granted the original came out before the "recession," but still.
Actually the Canon press release states that the MSRP of the 24-70L was $2099 USD at the time of release, not $950. And if you look, you'll see that pretty much all lenses Canon's released in the last decade have started out with a higher price than expected and then dropped 20-30% once the early adopters have bought theirs. Expect the new 24-70 to be at $1800-1900 or so within 18 months of the first shipments.
I wasn't talking about the press release, I was talking about what they were actually selling for when released compared to what they have been selling for recently. I'm just saying the lens was cheaper 6 years ago than it is now.
I'm sure the 24-70 II will drop below $2k fairly quickly, I was just trying to point out that lenses prices fluctuate much differently than most electronics. I didn't want the OP to get the impression that if he waited a year it would be $500 or something (like a computer or TV).