In terms of bodyfat%, anywhere from 9%-12% is where your abs start to show, if that visual helps. Most guards in the nba are between 6%-9%. Bigs have a little more. Ideally, I would think Cousins should get down to 12%. That would be in the same neighborhood as Duncan. I spent 6 years as a strength and conditioning coach if anyone is wondering where I got those numbers from.
Aside from that, Cousins has reportedly lost weight since the season, and is working out hard doing boxing/curcuit training/plyometrics. Not the same workout as weights. More for weight loss, explosiveness, quickness, balance, stabilty, core strength, and footwork. Apparently he looks much more toned now than he did during the season.