My daughter was 4 years old when I started with the "grounded" technique. She would get a warning of what grouded entailed and then if she didint listen that was it.
Her grounded was for a few scattered hours, she had to come home from school (5 oclockish) sit in her bed till dinner (6oclockish) and then go back in her room until bath (prob a 1/2 hour) and then bed for the night. She was only allowed to "read" in her bed. She has been grounded 1 time. It worked....now just the idea is freightning. This child is usually a very well behaved child.
However, my three year old is a DIFFERENT child all together......I just started with the threat of grounding (she will be 4 in Sept.). Her grounded entails no TV (she has a fascicantion with Strawberry Shortcake) for the whole "night" (which is about 3.5 hours from the time she gets home till bed time). She is stil very strong willed and will STILL fight or fuss. Sometimes if something "big" is going on over the weekend I will threaten her with it... IE if you don't stop your going to be grouned and not go to Lisa's b-day party...etc.
Of course if your child is 3-6 years old grounded should be short lived as they are not old enough to really "understand" the whole process. As they age tho grounding should increase. See how being grounded affects your child........then go from there.
Of course when I was 14 years old and acting a fool it was not uncommon for me to be grounded for WEEKS at a time. Good Luck Mama!
I think it depends on the child, the severity of the act and what can be constituted as being grounded.