How long will it take for a small sebaceous cyst take to go away on its own?

They don't. Sebaceous cysts, no matter the size typically don't go away on their own. If they are red and swollen they may drain spontaneously and appear smaller after this but will often recur. The only way to get rid of them is to have removed.
Probably never. Sebacous cysts have a lining which will generally make them recur unless it is removed. Therefore they do not tend to go away on their own.