There isn't really a set number for how many or how long they should take as far as I know.
The problem is that not only do you have to compete against bottom-of-the-barrel pricing, the type of person posting a contract in the first place knows this and doesn't want to spend a reasonable amount of money on a project. I actually got a couple of contracts that I had to end early, because of this.
Stick with Craigslist (I found nearly all of my contracts here) or a site like guru.com.
How does Guru set itself apart from elance?
An example I found (wasn't hard as it's filled with postings like this) is someone asking for a full fledged screen recording application (he has previously asked for a Java Swing GUI) to be delivered in 4 days for a budget of $25 dollars, he lives in the United Kingdom, and feels that someone residing in an ex-British colony should be able to feast on the earnings for years to come. What is even more ridiculous is the specification document is one sentence long. I doubt that this person would even take the time to write a paragraph of recommendation if someone unfortunately took that job.