Blinks tend to be a mystery to most animators. Some will just add a single eye closed in an animation for one frame. Others will prolong the closed eye for anywhere between 2 and 6 frames. The idea is that the speed at which the eye closes determines the reaction to the environment. A blink also gives life to a static character that is not being animated. This is usually a character that is in a scene as a background character or a secondary character that is holding still while another character is performing dialogue or acting. The blink may seem like a simple venture but in fact it is a little more complex than just covering the eye for a couple of frames. The use of an in-between (tween form) is highly effective in giving a more realistic performance as well. This in-between (tween) is simply a form that is in-between an open eye and a closed eye. The following are examples of animations done at 24 frames per second and demonstrate the comparison of both tweened and untweened animations. Use at your own discretion, but I highly suggest using tweens in your animation.
As you see the blinks are all very different from each other in the amount of time and emotional response due to the frame count and the use of tweens. Tweens give a better performance overall and also offer a more dynamic performance. My personal favorite is the 3 frame blink with the tween for standard character performance. It is just enough to not be overbearing to distract from the character and other characters. Longer frames counts can emit a little impatience or even patience. A faster blink can be used for shock in the moment performance. You can also modify these to be used in 12 frames per second performance by cutting down the time. So a 3 frame blink would be 1 frame of closed and in betweens on either side of the closed shapes. Example would be open, tween, closed, tween, open. This would give you a similar performance but at the 12fps frame rate.