I've noticed with my T3s it really depends on the juice. I have one (a dragon fruit) that darkens in about 1/2 a tank. I mean really darkens. By the end of the tank, it's practically coffee colored. Still tastes exactly the same, but wow, the color change is insane.
The juice I have now, from RAWR vapor, is still crystal clear, and I am 3/4 of the way through tank #2. I will probably fill it one more time before I clean the coil. I've run about 5 other flavors through it with no (or barely noticeable) color change. I'm convinced it's the juice.