It depends.
General Idea
Like TP said, 100 is a good place to start. I would be cautious with anything below, because you never know which way variance is swinging. When you get to 100, you will be able to gauge your ITM (in the money) rate and your ROI (return on investment).
Obviously, the more expensive the SNG buy-in, the more likely you're playing against better players. As a result, your "stats" or numbers for 100 $10/1's
should be better than 100 $30/3's. On the flip side, because $30/3's pay out nicer, you can still make MORE money even if you have a smaller ITM rate. Get it?
Keeping Records
It's extremely important that you keep accurate records if you take SNG's seriously. The math is not hard, but it's important. Most "extreme" SNG-ers spreadsheet everything, and I agree that it's a good idea. You want to document things like the buy in amount, the place you finished, any money won, how many players, etc etc. The better your records the more "on point" your stats will be. Be SURE to separate the buy-in amount. This is critical.
Calculating the Numbers
As I said, the math is easy, but it's important:
ROI: Profit / Total Buy-In's
Example: You've played 10 $10/1's.
You've placed 1st three times(!) but have busted out 7 times. You winnings are $150, but your total buy-ins are $110. Therefore your profit is $40. So, your ROI is $40/$110 = .3636, or, 36%.
ITM: Times In The Money / Total Played
Example: In the above example, you have an ITM rate of 30%.
Good ITM / ROI #'s
I've heard that good ROI's are near 30%. I personally think that number is very good, but who knows. If your ITM is around 40% you are doing well.
Hope this helps. I'd say your stats are solid when you approach 200 - 300 SnGs at each level.
GL.