How Long To Brine Boneless Chicken
It depends upon the strength of your brine, for boneless chicken breasts I recommend a 5% brine for 30 minutes to an hour.

Ideally, salt for brining should be measured by weight, especially since volumetric measurements for the same weight of salt will vary depending upon the coarseness of the salt. A 5% salt solution means you should use 20 times by weight the amount of water to salt. That's easy and foolproof if you have a scale. If you must use spoons or cups to measure salt, Morton's Kosher salt is very close to two tablespoons equaling an ounce, so 20 ounces (2.5 cups) of water to 2 Tablespoons of Morton's Kosher Salt would be a 5% solution. Use about 65% of the volume measurement if you must use table salt and measure by volume. 

Incidentally, for larger, bone in pieces brine in a 5% solution for 3-5 hours, a whole chicken for 12-16 hours (roughly).

Also, be sure that your chicken does not have water added, as many frozen, bulk pieces do. In effect, those chicken pieces are already brined.