Just a quick blog post about this topic since I was not able to find any good information myself before the D96 project began.
In my experience, when you open a bottle of D96 and develop your first roll in it, you have about 6 months of shelf life given that you try to keep the oxygen in the bottle to a minimum. I use Protectan spray which has worked well for me over the years when trying to extend shelf lives of various chemicals.
The developer will not change color, it was clear when I got the bottle (Bellini brand) and it was clear 1 year after. But after a year it was so dead you could not develop anything. I think the shelf life is worse than other developers (eg D76/Xtol) due to it being used as a stock solution. You will mix in a lot of oxygen into the developer when developing and then you pour it back. I get a over a year of shelf life with my XTOL in Protectan, but maybe part of it is due to me being careful when I mix it and when I pour some out.
D96 is a fine developer for cine stock, I think the negatives you get with Double-X in D96 are just perfect and have a wonderful tonal range. But given the cost of getting D96 pre-mixed, and it is less than stellar shelf life, I will not be buying it again. Next time I do D96 I will mix it myself. The recipe is quite easy and the components are not difficult to get hold of.
Anyways, please leave a comment if you have first hand experience with D96 and its shelf life!