I can honestly tell you, the first 3 weeks were the toughest for me. After that, Melissa is right, the sleep becomes more regular. And by 9 weeks, my son was sleeping 6-7 hours at a time at night. However, right now, it is tough. Babies have no concept of night and day when they are first born, and that is the toughest. After about 3 weeks though we would put him down at midnight, he would get up at say, 3:30-4 to eat, go back to bed after about 45 min. and then sleep till about 7am. And that I could handle. But, it just takes a few weeks and even then, it isn't always consistent. There will always be "bad" nights, for whatever reason.
As hard as it is (I used to feel guilty about doing it) but try and get help so that you can get some sleep, even if it is just a few hours. I use to feel like my son would forget me if I was away for a few hours sleeping, but he didn't! And it is important, or else you will go insane. Ha ha.