They don't actually mean "if it doesnt get you pregnant" when they say it didn't work - they mean "if it doesn't make you ovulate" which is really all it can do.
If on 50mg you have not ovulated then they would try 100mg to see if that makes it happen. If you are on 50mg and you DO ovulate, though, 100mg is not going to make you ovulate any better. That's what they always say. The best form of Clomid is Clomid at its lowest dose to make you ovulate. If you are releasing an egg with 50mg then 100mg will not change anything. It won't make you release more eggs, so to speak.
They normally accept defeat after six months, though, only because of the risks of cancer later in life.
Never worked for me. Never even ovulated on 100mg so I was never going to get pregnant on Clomid. It's something you have to try, though, just to say you've tried everything and for some it does actually work.