To summarize:

1. Firstly, you need to fly 100,000 EQMs for OZ Diamond (=Eqv Star Gold). This does not need to be within a particular timeframe (At least with the current rules) and so for all they care, you just have to fly 10,000 per year for 10 years and they will give you Diamond.

2. Once you requalify, then you have the 3 year requirement. In the 3 years following the qualification, you have to accumulate 40,000 miles, which some people do in mere months! so really, it's not very difficult to maintain.

The only stumbling block is that you actually have to waste 100,000 miles that can otherwise be accumulated into a more generous program, but if you are just after status, it is a pretty good deal once you have attained it. On the other hand, 100,000 miles is roughly equal to 3 years' worth of qualifying for AC Elite, which means you get to enjoy the status for 3-4 years in the same time you will otherwise be a basic member on the OZ program... so it's all about your priorities really.