From the AKC Cocker Spaniel Breed Standard

 The color of the tan may be from the lightest cream to the darkest red and is restricted to ten percent (10%) or less of the color of the specimen; tan markings in excess of that amount shall disqualify. Tan points in the Black or ASCOB variety shall be located as follows:

           1)  A clear tan spot over each eye;
           2)  On the sides of the muzzle and on the cheeks;
           3)  On the underside of the ears;
           4)  On all feet and/or legs;
           5)  Under the tail;
           6)  On the chest, optional; presence or absence shall not be penalized.

Tan markings which are not readily visible or which amount only to traces, shall be penalized. Tan on the muzzle which extends upward, over and joins shall also be penalized. The absence of tan markings in the Black or ASCOB variety in any of the specified locations in any otherwise tan-pointed dog shall disqualify.

Below are pictures of Black & Tan puppies with varying amounts and different colors of tan.  If you will look closely at the second picture (the puppy with deep rust colored markings), you can see that this puppy's tan points meet over the top of the nose.  This would be considered a "fault" in conformation showing and would be penalized. 

The puppy in the fourth picture is an example of a black & tan that might be considered to have excessive tan points (tan points in excess of 10% of the dog).  Some judges might disqualify this dog if it were entered in a conformation show, but most would probably let it show as the black coat will cover much of the tan when the coat is grown out.