At least 1-2 weeks for dummy campaign with the spend of ~50-100$, then I am starting main campaign with low budget, that I am increasing by ~10-15% every 1-2 weeks. So it's fair to say that it is about a month of warming up (dummy campaign + first main campaign which will be improved and expanded later + increasing budget to the right amount). Of course longer warming up usually pays off, but this is AdWords, sometimes you can be unlucky and have troubles even if you did everything correctly.