How long do the "terrible twos" last?

My daughter will be 3 on November 12th and the attitude she has sometimes! She'll have days where she doesn't do anything she's told to do, she'll misbehave at nursery when I'm at college, she'll snatch things off other children and doesn't say her pleases or thank yous...but then other days she'll be a complete angel. How long will this last and what can I do to stop her behaving like this?