My first DD would nurse for maybe 5 min. at a time and be done for a couple hours. She was my "efficiency" nurser. This little one eats a lot more often...not necessarily for longer periods of time (although she will when DD wouldn't) though.
Mine nurses for about 5-10 minutes on one side. I offer the same side again if he wants to nurse in the same two hour period. I've done this with all three and it works well. My first was a real snacker, always fell asleep while eating. When he got to about six months I started doing more scheduled feedings rather than on demand, and that helped him eat more efficiently and helped his tummy. I wouldn't recommend that for a younger baby, though.
Depends, really. Sometimes, she'll nurse for just 5-15 minutes on one side. Sometimes, she will nurse for another 5-10 on the other. She pops herself off the first breast when she is done, and I always offer the other breast. Sometimes she takes it, othertimes she does not. Unless we are going through a growth spurt (and we're due for the 6 week biggie in 2 weeks ...), she nurses every 2-3 hours during the day, every 2-4 at night.