Beware the dreaded plateau

I was reading the Shape Magazine, and it mentioned that:

"..if you recycle the same exercize regime, the dreaded plateau is bound to develop"

 

Now, I can just imagine the fitness bunnies giggling amongst themselves and knowledgeably agree that the plateau is indeed unfavourable.

 

Me, on the other hand, I am thinking: What the heck is it? Sounds frikking awful, is it contageous and is it the reason I don't look like the supermodel I am destined to be?

 

I know that you cannot blindly follow the same exercise for a long period of time. But then, the question is, how long can you stick to a programme at a time, before it becomes a routine to your body?

 

And another life long mystery is whether exercise must hurt for it to be effective? Do you have to be stiff the next day?