Varies a lot...
Boils come in many sizes, such as 1/4 of an inch up to more than an inch.
They can be anywhere in the body. Also, different people's bodies react to boils differently (even more so if they are suffering some disease or taking certain medications). In general, it should take 2-3 days, but a doctor should check it out if there is any reason to worry, of if the boil isn't merely a tiny one.