How do I gauge when my son has been given enough swats -- I have found as he gets older that he tries to "macho" through a spanking and it has become harder to tell when he has been chastised sufficiently without crossing the line into abuse.
A parent should look at the discipline being delivered. Does it fit the crime (is it a good consequence related to the misdeed)? Is it being delivered in a timely manner? Is it suitably severe enough? Even when the answers to all these questions are honestly "yes," but the child "laughs off" the punishment, deliver the punishment anyway. The constant annoyance of being punished will break through the walls of the most stubborn child. The parent should show more patience in delivering a needed consequence than a child in receiving the consequence. This is the point of Proverbs 26:3, "A whip for the horse, A bridle for the donkey, And a rod for the fool's back."
Proverbs 26:3 is dealing with control or the giving of direction. The rider of a horse can give directions to his mount with a small horse whip. Even though the horse is much bigger than the man and the strikes of the whip are not much more than an annoyance, yet the horse will comply with the rider's will. Donkeys are notoriously stubborn beasts, yet a rider can direct a donkey with bridle about the donkey's nose. Again, the donkey is stronger than a man and more stubborn than most men, yet a small tug with a bridle will win out in the end. As with the first two, so is the last. A spanking with a switch (rod) on fool's backside will cause him to give up his foolishness and be obedient. The pain of being switched seems to such a minor annoyance, yet it can cause a change in behavior. Just as a horse whip is well-suited for directing a horse and a bridle is well-suited for directing a donkey, a switch is well-suited for correcting a fool. There is also a more subtle statement being made as well: fools are little more that brute beasts in their behavior.
Another verse to consider is Proverbs 19:29, "Judgments are prepared for scoffers, And beatings for the backs of fools." This is dealing with the certainty of consequences. Scoffers are fools who scorn authority, but their disdain for authority will not prevent them from facing a judge when they break the law. In a sense, the law with its recommended sentences (judgments) were written for people who believe they can ignore the law (the scoffers). In the same manner, fools act without thinking of the consequences of their actions. Just because they believed there wouldn't be a consequence, it doesn't mean they won't receive one. The use of spanking the backside of a fool is laid out in advance because fools exist, doing things without thinking. This verse is comparable to "For rulers are not a terror to good works, but to evil. Do you want to be unafraid of the authority? Do what is good, and you will have praise from the same. For he is God's minister to you for good. But if you do evil, be afraid; for he does not bear the sword in vain; for he is God's minister, an avenger to execute wrath on him who practices evil" (Romans 13:3-4).
For parents this means that in advance you decide what is the appropriate punishment for a particular misbehavior. Not all misbehavior calls for a spanking. I generally recommend that parents reserve spankings for violent behavior or willful disobedience, especially cases where another punishment that is tied to the misbehavior cannot be found. If spanking is called for, decide what number of swats are called for in advance. Each incidence receives the same number of swats. Do not gauge the number of swats by the child's reaction. Instead gauge it by his behavior over time. If it appears that spankings aren't slowing him down from one incidence to another (i.e. that over time it is not causing a modification in behavior), then increase the number of swats a bit. However, what usually happens is that when a child misbehaves frequently, he is going to receive a frequent number of swats. I haven't met too many boys so thick-headed and thick-skinned that this doesn't eventually makes an impact on their behavior, though perhaps at first you might feel like you barely finished one spanking before another one is called for.