1. When you do head flick - in term of choreography?
2. When you do head flick - in term of musicality?
A "head flick" can mean a number of things, so it's probably best to clarify exactly what action you're thinking of. Any quick turn of the head might be described as a head flick, which can include everything from a simple turn to promenade position, to a Spanish Drag with an elaborate combination of multiple turns of the lady's head back and forth.
If you're thinking more broadly and generically, then a "head flick" is can be thought of simply as a quick turning of the head, typically, but not necessarily from closed position to promenade, or vice-versa. In this case it is nothing more than an interpretation of an action that is already present in many basic figures such as the Link, Closed Promenade, Closed or Open Finish ended in PP, Outside Swivels, and so forth. Specifically, the interpretation is one that is both "sudden" (i.e. occupying as little time as possible) and "bound" (i.e. movement that is contained by way of increased muscle tone).
Please note that the concept of "sudden" movement, while it does imply quickness, does not necessarily relate to the timing. You can, for example, have a turn to promenade that is meant to take place over the course of two beats, or one "slow" count, and still interpret it in many different ways. To illustrate a movement that is sudden, you would do it as quickly as possible, say at the very beginning of the first beat, taking as little time as possible to complete it, and then waiting through the remainder of the two-beats -- This is still considered a "slow" from the standpoint of musical timing, even though the movement itself is sudden. Compare that to the opposite interpretation, one that is "sustained", where you would use the entire span of two beats to complete the turning to promenade.
This is an important distinction because it illustrates how you can be prescribed a certain timing, say for example QQ for a link, or SQQS for Closed Promenade, and still interpret the turning of the head in various ways: sudden and bound, or more gradual and relaxed, or perhaps somewhere in between. The "flicking" of the head, then, would be the appearance of quickness and bound-ness when turning from one position to another.
If you have something more specific in mind, let us know and we'll try to figure out what it is. Chances are good, all of the information above will apply. A very popular movement you might be thinking of when you say "flick" is one where a couple, already in promenade, will turn briefly to closed position and immediately back to promenade, all while standing in place. I've heard it called a Shrug, but it might go by other names as well. It is often done to the counts, "and slow", tacked on to the end of another figure, to complete the last two beats of a musical phrase. For example, if you have something that ends in promenade on counts 5,6, (e.g. two walks and a link), and you don't want to sit stationary through 7,8 to start the next movement on the next count 1, you could fill the time with the extra head flick on &7 (hold8).
3. Is there any limit for head flick in a dance? (min/max of head flick times)
As with anything in dancing (and in life for that matter), most things are best in moderation. There are no absolute rules, but you don't want to go crazy with any one action any more than you would with any one step or pattern. So, just like you do with your choice of choreography, just remember with your artistic interpretation to keep things interesting. Mix your suddens with sustains, look for opportunities to execute actions both bound and free, and try to do it in ways that emphasize the character of Tango. Remember that Tango is not all "attack"... There's all the time you spend working up to it, while stalking, slithering, pursuing, seducing, etc.
Regards,
Jonathan
