I had four solid long-term contracts lined up as of a few months ago, scheduled so that I could make adequate progress on them without overloading myself, with the overlap set up so that I'd be protected if any one of them fell through.
So far, two of them have fallen through (in the non-paying fashion) and one of them is in the process of actively falling apart. I ended up sinking a ton of energy into trying to salvage each one and trying to protect clients from themselves, but ultimately it's not possible. So now I'm potentially left with one paying project... if that one falls through I'm screwed.
Maybe in some fields you can be looking for work and have some good contracts in your lap a week later? For me, jacquesm's '3 months or so' is on the money; some of my better-paying contracts take even longer than that to get to the point where I can actually start doing work and billing for it.