I can't beat the GP7 wait time.
I think my top active one right now is the Lionel Milk Cars that got held up due to tooling issues and changing the country of manufacture. According to my records, those were 2015 Volume 2. But I think they are now scheduled for roughly the end of this year.
So a little over 5 years for those, but that's an extreme example, and I give Lionel credit for keeping up the commitment to making them. They could have just abandoned the line of cars once the original manufacturer was no longer viable (or whatever the full story was, I don't know the details, just that it changed).
That's getting bad when you wait so long your credit card expires. I have seen many items on vendor websites that are still listed as pre-order from 2017-18-19 catalogs. I've looked for an official list of "decided to never make" items, but have not found one.
Several price guides (you know, those things everyone now says are useless due to eBay) used to cover that relatively well, but that was when there was a whole lot less product offered, and there were also fewer cancelled items.
If you look in a guide, you may well still see it for things from the 80's into 90's (maybe even a few more recent), "NM" = Never Made, and then there are of course no prices (aside from originally intended MSRP, perhaps) since there can't be any reported sales for things not made.
Unfortunately it's impossible to run an eBay query that will tell you the thing you are looking for doesn't exist....
-Dave