First, congratulations on 2 months of lifting! That's a fantastic start.
On to your question: the BF% at which women start to see muscle definition depends on the body part and a little bit on idiosyncracies in where the woman in question stores fat. The good news is that most women see definition in their arms and back before legs and abs. A ballpark BF% -- and this is just a ballpark -- is about 20-22 BF% for arms. By contrast, I don't see my abs until about 14-15%. (It's also a hard question to answer because there's seeing abs in the right lighting and pose and when the wind is from the northwest, and there's SEEING ABS -- it isn't an all-or-nothing affair.)
BUT, and here's some unsolicited advice (which you're of course free to ignore!), you may need to reorient your weight training in order to get there. You don't say what lift you use 15 lb dumbbells for, or what exercises you're doing on the machines, but generally speaking (a) free weights are better for building muscle efficiently than machines, and (b) you get far more bang for your buck from doing compound lifts that work more than one body part at a time than you do from isolation exercises (bicep curls, most machines). To learn more, check out threads on Body for Life or New Rules of Lifting for Women (both books/programs. Better still, and free, get thee to Krista Scott Dixon's site for regular women lifters at www.stumptuous.com