It's written into most contracts as 2 weeks, occasionally 1 or 4. If you can pull it off, offer 4. No need to go longer than that. Start organizing yourself before you had notice.

At the bank I mentioned earlier, someone gave notice, and was immediately walked off the floor by security. He had organized his work completely so that it was an easy transition despite him not being there. Everyone had good things to say about him afterwards. (And he got paid for the notice period, which is standard if someone says, "Don't bother coming in.")