It is timed. I can't remember exactly how much, but I had more than enough time to go through the questions twice and then back over any marked ones for final checking. I think I had about 30 minutes left at the end and I took my time.
You can start it as soon as the payment goes through, but you are given an amount of time that it will be available, which I think was a couple of days from the point of purchase. Obviously as soon as you start the exam, that is it!You can't pause for a day and come back later. Which I could have done with as my wife and kids returned home halfway through mine.
As its an online exam I am not sure what the rules are with regard to having notes or reference books. I mean there is no-one there to stop you! Most of the questions were structured in such a way that having a few notes wouldn't have helped anyway.
I would say brush up on everything, especially any weak areas you had when you sat the CS5 exam as you never know what will crop up.