There is no such trade-in program for iPod touch from Apple. Well, except for the 10%-off new iPod recycling incentive if you bring an iPod to an Apple Store and buy a new iPod.
https://www.apple.com/recycling/ipod-cell-phone/
It's possible a third-party electronic refurbishing company will offer you something for a used iPod touch, which you can apply toward a new iPod touch.
This is Apple's user discussion forum. No one posting here knows when Apple releases new products. However, "history" says it was almost three years between 5th gen iPod touch and 6th gen iPod touch (released July 2015). it was about two years between 4th gen iPod touch and 5th gen iPod touch.