When a contract says "6 months" that's how long they expect the work to take, but bear in mind that usually you'll be on a 1 or 2 week notice (sometimes longer but rare) so it doesn't really mean anything. The reason contractors can command higher rates is that we're a lot easier to get rid of if the work dries up or priorities change - fortunately I've not had this happen yet.
On the positive side if you're decent most places seem keen to keep you on, so that initial 6 months may turn into 10, 12 or longer. I know guys that have been at places for 2 or 3 years but it starts to look a bit suss with HMRC once it gets to that level. After a few contracts you'll hopefully start to have a network of people that are happy to suggest you via word of mouth before roles even get offered on the job boards.
The hardest bit is starting out - I had a 3 month notice with my previous permanent employer so I just had to bite the bullet and hope I could find something with a couple of weeks to go. Best thing to do is check to see how frequently contract roles come up in your area for your skills - if there seems to be stuff around all the time then go for it, but have at least 2 months of living expenses saved up as cover. If worst comes to worst, you could always go back to perm.