There seemed to be plenty of time for the exam - It was between 3 and 3.5 hours, I forget exactly how much time tho.
It is possible to revisit questions you've already answered, and answer them out of order. You 'mark' the ones you left blank, and it will let you review them at the end to change the answer.
The drag and drop questions, if you revisit them, will totally wipe your old answer! So it's best to not revisit them unless you were unsure before.
Your preparation might be sufficient, it depends - I assume it's the SCJPtest you're referring to. Did you do all the sample problems, in addition to reading the book? If so, then your preparation is what I did, and I passed with 85%. It took a while to go through that book, but making my own notes helped a lot - I simply reviewed my notes the day before the test, and all was well.