The longest one of my sons was grounded was for 2 weeks, along with no TV, no video games or computer. The infraction warranted it! (Disrespect) to his teacher.
Don't you think, however, that taking beloved gadgets (nooooooooooooo!!)
away was enough to get the point across?
Personally, I have doubts about grounds being efficient...
For us there is another element of 'grounding'- we use the time to rebuild or build further on family relationships. We don't let grounded kids sit by themselves in their rooms being sullen and building their resentment. They get less time without outside influences and more time with parental influence.