how much time before you are forced to buy a new Mac? I understand that apple provides OS updates for 7 years but how many years after this before you are kinda forced into buying a new Mac to support new apps and new updates of apps?
No one is “forced” to buy anything. There are many people using 10+yr old Macs happily… with original OS and using original applications that work well on those systems. The decision to buy a new Mac is an individual decision based on individual wants and needs.
It really depends on the whims of specific developers. If there's some app that's really important to you and its developer is aggressive about moving it forward then that could tip the balance. But overall the MacOS app scene seems quite a sleepy... ahem, chilled out... place.
As of right now only monterey, ventura and sonoma are supported. As long as the app and macos are within compatible range then its all good. You are only forced to upgrade when you want/need the New feature. normally ony 3 macos are supported at one given particular time, and the only time ive seen it any different was with mojave, catalina, big sur and monterey.
Yes, for the operating system and some apps. For a 2012 MacBook (Air or Pro), Apple only runs macOS up to Catalina (10.15) which is no longer supported even with security fixes by Apple. Looking at apps, the current version of some apps may not be supported on that 4 year old version of macOS. For example, Adobe Photoshop requires macOS 11 (Big Sur).
The situation becomes more messy if the owner of the 2012 Mac wants to continue running old software (e.g. 32-bit versions) and therefore is not able to update macOS to Catalina. That means that more current apps will not be usable on that Mac.
Assuming you have a specific Mac in mind, what version of macOS is it running and what apps are of concern regarding support?