Wrangell is spot-on....
There are several measures you can go by...weight, body-fat percentage, BMI, etc. In the end, most people will see some serious definition around 10% Body-Fat.
If you really wanna focus on a scale figure, I'd guess (shooting from the hip) that you'd need to get down to about 162-167 pounds to start getting the visual cut-ups. But it depends on your build. And let's not forget, as you continue to do weights you'll put on muscle....you could get all ripped and still weigh 185, so working with Body-Fat Percentage is your best way of gunning for a numerical goal.