This is what I know from seeing actual beretta M9's. They rate the Barrels at 20,000 and the frame at 35-40.000. That is a life time of shooting unless you shoot competitive or multi tour combat use where you are training and then deploying and using the same gun for everything. More times than not, locking block issues with the barrel would end an M9's life than the real wear of the barrel itself. Now these numbers are a guideline.
Every year in 3rd SFGA no matter the the amount of rounds fired or stress of the gun weather it is M9 or 1911 the armors did a complete rebuild of the gun. This was hammer, trigger, Sear (fire control system), firing pin, all springs, and mags rebuilt (if after market). Now we put a lot of undue stress on our pistols that the average person is not really doing. From training with blanks to just carrying our full load and doing Airborne ops where the gun is getting landed on ran through dirt, and then being slept with while it is pouring down rain in NC.
Some stuff would break or wear out and be replaced of course at due time, but as a rule once a year, over haul and follow the factory guidelines for frame and barrel wear
You may not need to replace parts a like this every year but if it is your self defense, number one shooter, every six months you should be doing a full inspection with a functions tests weekly and every time you field strip or break down your handgun.