Best time to buy a used iPhone 6?

** EDIT: sorry, just noticed this is basically a repeat post. **

Hi! I'm looking to buy an older model iPhone (in my case, most likely the 6 Plus), and I'd be happy with a used phone in good condition. I figured it definitely made sense to wait until after the release of the 6s, but I was wondering if any one has a sense whether to buy now, or wait a bit longer? I'm sure the price will only continue to go down over time, but I could imagine either a sharp fall-off followed by a shallow one, or just one long shallow fall-off. In the first case, there might be an optimal amount longer to wait before buying. I just wondered if someone has a recommendation from past experience.

Thanks for any advice on the matter, sorry for the verbose post . I'd also be glad to get any general advice on buying old iPhones, but I'm sure there's a lot of info on that available already.