I have a 2008 CLS 550. My lease on the car will be up on July 1.

I wanted to find out what your experience has been with Mercedes regarding tire wear when you turn in the vehicle at lease end.

I can't remember - is there a stipulation that a certain amount of tire tread must be remaining - like 1/8" or is there no stipulation regarding a minimum amount of tire tread that must be remaining when you turn in the vehicle?

I don't know of MB's specific tolerances but all manufacturers have a limit beyond which they will require you to replace the tires. Check with your dealer or wait for your pre-lease end inspection and you'll know whether to replace the tires or not.
My BMW just went back after 3 and a half years with just over 25,000 miles and the tires were within the acceptable wear.

The good news is if you must change the tires there is no rule as to what they have to be, so you can go get a set of Pep Boys specials!