The higher up one rises, the less accurate this "average" is. For example, at junior grades, promotion generally happens every 2 to 4 years since these promotions are not linked to appointments. But at DD2, some people will take forever to reach DD3 since higher DD levels are for division head appointments that are limited by vacancies and hence only for the identified ones. This indirectly affects promotion to lower grades too since of course can't have everyone stuck at DD2.

So, the answer is: It depends on your CEP and if you are identified for the leadership track, maybe 2 years. If not, maybe 5-10 years. It usually takes longer and longer to get promoted (just like everywhere else), so if you took 5 years to get from AD2 to DD1, don't expect any shorter than that from DD1 to DD2. If you just joined at DD1, your CEP (and promotion rate) will depend on your performance in the first couple of years.