The key to success for any approach to discipline is that there be consistency, and that punishment should be certain and predictable. There is not a simple answer to the question of when a parent should spank their child, but here are a few things that should be kept in mind:
1. Except in the case of the most egregious acts of defiance, a child should be warned in advance that if they cross a particular line, specific consequences will follow.
2. Where ever you draw that line, you should follow through with the threatened punishment without fail when it has been crossed.
3. You should not spank a child in the heat of anger, but should take a moment, and say the Jesus prayer before you spank them.
4. When you spank a child, it should inflict enough pain that the child will not want to cross that line a second time.
5. The older the child becomes, the less effective spanking will become, and the more you should use other forms of discipline.
One of the problems many parents have is that they threaten punishments, but too often do not follow through. Consequently, the threat is not taken seriously. Then when the parent get's really angry and frustrated, they finally spank the child... but little is gained by such an approach, because there is no way that the child could have predicted what would finally trigger a spanking.
Children will test boundaries to see if they are real boundaries or not. If you mean what you say, and are consistent, you will save yourself a lot of frustration, and you will also avoid confusing your child by your inconsistency.
A good author to read on this subject is Dr. James Dobson, and the best book to start with would be the classic "Dare to Discipline."
