Garands like most military rifles are ruined by improper cleaning, not shooting.
If you shoot ammo designed for the Garand (another topic), keep it lubed it will take dern near forever to shoot it out.
I got my CMP (then DCM) Garand in 1981 or so. I've been shooting it quite a bit since. I use to run sniper schools for the NG when we used M1C/Ds. During the schools, many times I've shot my Garand until the sap boiled out of the handguard.
I checked the Muzzle and it measures just a hair short of "2". The Chamber gages measures right at "8". According to many it should be shot out. Below is a target I've shot reciently. I shot it from the prone positition, no support other then the sling. As you can see, if I'd came right one click, and up a click, and tightened up my position a tad, I would have cleaned the target. This is the 100 yard reduced 200 yard GSM target.
What ruins Garands (and other military rifles) is cleaning from the muzzle with a jointed cleaning rod, or coated cleaning rods. The coated rods get grit and carbon imbedded in the coating which makes it like a file.
On any rifle I only use one piece stainless rods. For rifles that require cleaning from the bore, I only use bore snakes.
Shoot the rifle as much as you want. You wont hurt it, I bet our fathers during WWII and Korea fire a bit more then 700 rounds.