Really, it's not fair to generalize. I'm speaking from my own experiences and what I've heard from friends and colleagues. The decision is very much an individual one, with more than pure academics to consider. Similar to choosing MA, or MA-Ph. D. programs, the length of time it takes you to complete your degree varies tremendously.
Anthropology requires research on humans, primates, or artifacts.
Data collection isn't something you can do with that much alacrity. Many of my classes were also non-anthropology classes, too. The Ph.D. varies even more widely. It also depends very much on the nature of your research project. Again, data collection is not something that can be rushed.