Probably a long time, but thankfully questions can be edited as well as answers so even a casual contributor can help to repair question wording, and make answerable questions out of unanswerable ones.
Stop asking unanswerable questions.
no.
WikiAnswers!
There is little that can be done to prevent stupid people asking unanswerable questions.
Questions seem unanswerable for various reasons -- there may be spelling problems or vague grammar. Then another contributor might come along and realize what the questioner was really asking, and offer an answer. Don't forget to correct any spelling or grammar errors in the question itself, so that others will have an easier time finding it in searches.
I would guess that they are just "killing time" by asking many questions just to see what/how many reply they get.
No. But asking stupid, pointless questions on WikiAnswers is.
No, you can also answer people questions.
Basically, yes.
Some questions on WikiAnswers are of no importance to the person asking and the person answering. Ask legitimate questions for legitimate answers.
For sharing knowledge by asking and answering questions.
As many as you want and are capable of asking.