Takes me about an hour. Unless I am doing something, like yard work. Then it seems the bowl is gone in 20 minutes. When I am just sitting and smoking, I'll often (gasp) let the pipe rest for an hour or two, or the rest of the day, and finish it off the next day.
I've found letting the pipe set in the bowl for a day or two sometimes helps the tobacco burn better. I suppose in that case the tobacco was to moist to begin with.