What doesn't kill you, just makes you stronger

How many times have I heard that one? 

Is it true?  Is it really, really true?  I'd like to know. 

And to make you stronger, does it have to feel like it's going to kill you first? 

And how much stronger does it really make you? 

Because there are times when it really feels like I won't survive,
but how soon after should I start to feel stronger??? 

I really need to know.