The stat you want is ROI. Return on investment. How many times you cash is not the best way to judge your SnG game.
ex. if you cash is say 3rd place 10 out of 20 games and win $1 per cash that's $10. But if you cash in first only five times and get $5 per cash that's $15.
You cashed half the time but made more money.
The ROI will be different dependinng on the level you play. You can check out sharkscope
to get a idea of what the rate the winners have at your level.
Just a note. You will want to look at how you are cashing though. If say as in the example above happens - winning 10 3rd's and no 1st then you may want to figure out why you are not making 2nd or 1st, ie..bad end game?
Best of luck.