well.. none. it will always age and get slow because games will get more demanding. that is just how it is. how long before you need to upgrade? well it depends more on the user than the hardware if you ask me. As long as you are happy with how it plays, then you won't need to upgrade unless something breaks.
some people upgrade even if the hardware is just 1yr old.
Could you ask this question every day? It is
THE MOST IMPORTANT OVERLOOKED ASPECT
of building a gaming pc.
Without your motherboard specs, I would have to guess;
Prebuilt intell rig; 44 months to doom.
Prebuilt AMD rig; 39 months to doom.
O.E.M intel rig; 60 months doom
O.E.M. AMD 45 months doom.
Homebuilt , quality parts (asus)
intel 90 months
AMD 55 months.
See worst case scenario's.
Homebuilt (gigabyte )
see asus ,+ 7 % ongoing critical hit fail chance.
see asus, less frills ,+5 % later critical hit chance, if critical hit missed
longevity will ensure redundancy.