It depends on the size of the chicken. I've weighed a lot of chicken over my decades of dieting, and I'd say that 7 to 10 ounces of meat would be a pretty good guesstimate.
I know it's a pretty wide range, but chicken piece sizes really vary a lot. I've had breasts that barely yielded 3 ounces of meat, and others more than twice that.
If you can't weigh, a decent visual guideline is that one ounce is the size of your thumb, and 3-4 ounces is the size of your hand without the fingers.