Personally, I only check gravity before pitching yeast for initial, and before pitching corn sugar for bottle conditioning, for final.
Takes a significant amount of your 5-gal batch if you check more than that. What you learn from repeated samples IMO won't be as valuable as another bottle in the box. I can see from the bubbles in my blow-off tube whether its done fermenting (which it is after only a few days).
Also, I highly doubt the krausen is still there 13 days after pitching your yeast. It probably just looks like it because of the foam debris left on the inside of the fermenter... If it does still have krausen... then I guess it's not even close to done yet.