Hello vlsi,
You can always write to the school and ask them if sending scores late is ok with them. Most of the schools do accept GRE scores after the deadline, so it shouldn't be a problem. Better to ask the school.
Well, if you got a good score, it doesn't harm to send your scores. Again, much of this is university dependent.
About the time frame, it is a very subjective question. It depends totally on you. If you're a CS major, you'll require less time. You may prepare in 2 weeks, or you may take months. So,.. thats it.
hth,
Vipul.