For Fairphone’s default operating system (now called “Android OS”) @Meaghan already linked to the roadmap.
While Fairphone will not upgrade it to Android 14, alternative operating systems for the the FP3/FP3+ have already upgraded to Android 14 and 15, see
This might allow security updates for 2-3 years more (my estimate based on Android version history - Wikipedia), given you are willing to install one of these.