There is no official timeframe that Apple hardware will support the latest operating system or versions of particular software. It will vary depending on the type of enhancements that occur in hardware and what impact this has in terms of what the latest software needs in order to be able to perform certain tasks.
The person who told you that Apple has an "8 year support window" may have been getting confused with Apple's policy on Vintage and obsolete products, but this relates only to hardware/parts support and has no implication in terms of the operating system and other software.
- Vintage products are those that have not been manufactured for more than 5 and less than 7 years
- Obsolete products are those that were discontinued more than 7 years ago.
However, even the above will have some exceptions depending on local laws.
Using macOS Sierra as a guide, it is officially supported on:
- iMac: Late 2009 or newer
- MacBook and MacBook Retina: Late 2009 or newer
- MacBook Pro: Mid 2010 or newer
- MacBook Air: Late 2010 or newer
- Mac mini: Mid 2010 or newer
- Mac Pro: Mid 2010 or newer
As you can see, in some cases support goes back to 2009 models, while in others it goes back to 2010 models. Therefore, there is no official timeframe that Apple hardware will support the latest operating system or versions of particular software.