I took 4 weeks off the first time and it was too long. There is very little for the dad to do the first few months and really I was just kind of in the way. There were tons of relatives and friends in and out for the first month or so, and my wife and baby were either just breast feeding or sleeping. My wife even says that it would have been more helpful to take the time off when she was planning to go back to work- for the adjustment period.
The second time I just took a week off and it was perfect.