I have a Sturmey Archer X-FDD dynohub and drum brake for about 2 years. I've riden it about 6000 miles and I don't know if it is due for maintenance or even what kind of maintenance is required. I can't seem able to find anything about it on the interwebs. Any pointers?
There is very little to do with this hub.
Mostly you need to tighten brake cable.
After 2 years of everyday use with 7-30 kilometers per day in city traffic with lot of junctions, using front brake as main brake (est. total 10 000 km) in my case there is still a lot of ( ca. 60-75%, didn't measured exactly) brake lining on my X-FDD brake shoes.
For most regular users it's going to be 'lifetime' with no maintenace.
I disassemble it every year, what is, in my opinion, more often than required.
Sealed bearings require replacement when they get perceptible clearance. Brake shoes need to be replaced when brake lining is gone (you will hear & feel the difference).
What I do with it is also greasing the cam that pushes brake shoes (1) and axle on the opposite side (2) (see picture), that keeps shoes together (it's inside and need to be done carefully so as not to grease braking surface). This procedure reduces friction and make you use less power to pull the brake lever.
The second problem may appear when you like to pull lever lightly when approaching a junction slowly. It produces a glazed surface on the brake shoes, that makes braking harder, and less effective. You can get rid of it with sandpaper, but it's easy to break the circular shape of the shoes. Less effective, but a safer option is to brake firmly when going down the hill - several times.