It might be more than one single every 5 games, if you are talking about the difference between Leon & Swihart. It might be a few HRs a year, more doubles, singles and walks. If it is a double and a walk every 20 PAs, it would be like adding over .100 to Leon's SLG% and .100 to his OB%. That's over .200 in OPS (added to .550 is .750). Now, that might be high end estimates, but it is an example of how much great catching can make a difference.