Currently I'm using "MK8 Hardened Steel Nozzle 1.75 mm" for my Bowden extruder 3d printer. And I have no exact idea how/when do I need to replace my nozzle. Although I usually change every 3- 4 month. During that time, I 3d print nearly 1Kg - 1.5 kg of filament. 

Do you have any idea, do I over replace the nozzle or how exactly determined when do I need to replace the extruder nozzle and what properties do I need to consider on this matter?