Chain Adjustment

How often does the chain normally have to be adjusted? I just noticed my chain slack is out of tolerance. What is the factory chain on a CRF450R '03?

I have a DID 520 ERT chain on the bike now. How good is this chain? Bought the bike used, so I am learning everything yet. The chain was measured and the slack was correct when I first bought it and did an overall inspection.
What is a good chain if this one is not? I heard the O-ring type hits the casing. Why one vs. the other?

I have done lots of research on this now. I was shocked how much my chain stretched. I now ordered a chain with a very high tensile strength of 8800 lbs. We'll see how long this last. New steel sprockets ordered also.