DeathStar-Alpha.........
The current D Cat is basically designed for Airline Pilots, Ag pilots etc, so they can do type ratings, also IF ratings if you are IF rated.
The current D cat does not expire, but you must have a current licence, and also, at least 700 hours on type.
Changes are coming re 'renewals' just like a standard intructor rating.
The Two D cats I trained a few years ago, wanted them for specialist type ratings, in both cases tailwheel flying.
Neither wanted to do C cat, as they were only interested in their specialist field, and both are very good at it!
I got mine, cause I have so many other bits and bobs on my licence, I thought it would look good.........pure vanity.........
I do all types of training, so I dont need the D Cat anyway........
The CAA rules are very hard to read, as they are very poorly written, so
its very hard to actually understand whats what.
Cheers