Bro be careful what you are saying, imaan is in the heart not on your forehead. You think even IF you have achieved the BIGGEST bump on your forehead that you cannot die as a disbeliever? Look at Iblies, he was WITH the angels based on his worship, however we all know where he will end up. Forget the bump, rather look at your heart.
Or even doing the prayer not for Allah, but to achieve the bump. It will NOT be counted as prayer as it was NOT intended to worship Allah, rather to achieve the bump.
Of bump: if you mean black spot on forehead due to prostration then it depends on your piety and it takes a long time or if your forehead is bulging out then within a week.
I can give you one in half a second, it may take a few minutes to show, but it'll be pretty, I promise - all you have to do is close your eyes and stand very still,