Leaving a magazine loaded, even for extended periods of time, shouldn't adversely effect the magazine.
That said, M9 mags, seem to be POS and require continual/often cleaning/maintenance.
I've had glock and H&K pistol mags loaded for years with no issues. I would never try that with my M-9. I clean its mags once a week, and verify function.
"If there must be trouble, let it be in my day, that my children may have peace"