In practical terms, how often do you have to adjust the belt tension of a belt drive bicycle?
I have been riding a Gates CDC (not Centertrack) belt drive for almost a year now.  Gates claims that tension adjustments are not necessary over the lifetime of the belt drive.  My experience does not support this claim.

One of Gates' claims is that the belt does not stretch enough to warrant tension adjustment after a break-in period, as you would on a brake or shifter cable.  I found this to be true.

However, I have noticed that the tension of the belt changes significantly with temperature (and possibly humidity, though I don't have enough data to test that hypothesis).  I have ridden my bike in various weather conditions and temperatures ranging from over 90℉/32°C to under 0℉/-17°C. I use the Gates iPhone app to check my belt's tension.  I have noticed that, if the belt is tensioned properly at about room temperature, then the tension in the belt decreases below the recommended tension range when the ambient temperature drops below about 20℉/-6°C.  I have noticed problems with the belt drive when tension is too high or too low, including excessive noise, skipping, and misalignment leading to dropping the belt from the sprockets.  Therefore, I found it necessary to retension the belt slightly in the beginning of the winter and again at the end of winter.

I'm not certain yet if the weather-related change in tension is caused by frame shrinkage (I suspect this is the case - my frame is aluminium) or by another factor.