It should last a loooong time if you just use it on your scalp. It would probably last me a year. But if you put it all over I would say about a month.
No it is a fairly large bottle, about 12 oz. Let's see, I have bought about 4 bottles since June (I think), that includes less than half a bottle I gave to my boyfriend's mother.
I have been using it about for three weeks, once a day, and I have 3/4 of a bottle left. So one bottle is likely to last me 12 weeks. I think that you use less when you apply it to wet hair. But, I think I prefer using it on dry hair.