If I marry a Brit, will I become a UK subject righ away or it takes years. I am from Finland?

First of all you wouldn't become a subject you would first become a citizen. Secondly no you would not automatically become a citizen you would have to apply although marriage to a british citizen would make the application a breeze. After you attain citizenship you would the be considered a subject of the queen ( kind of a demeaning status in 2009)