The simple answer is you shouldn't keep anything with a snapping turtle you don't want to be eaten, or harassed. The next question I would answer is my common is nearing 10 years old, and is close to a 12" carapace.
I always suggest this site for good advice http://www.chelydra.org/snapping_turtle_serpentina.html
I always suggest this site for good advice http://www.chelydra.org/snapping_turtle_serpentina.html