How long does it take to the body to get used to a type of exercise?

I have started running and I got to the point where I run every day. I've read that if you do the same type of exercise every day, your body will evetually get used to it and it won't be as effective as it was before. So, should I do different types of exercise, or stick to running, but increasing the intensity or distance?
How long does it take to the body to get used to a type of exercise?
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