I just pulled apart my Shimano dynamo hub according to the instructions from Trinity Bicycles. These are loose ball cup and cone hubs, but are technically "non-serviceable" according to Shimano, because there's some complicated disassembly around the drive side plug area.
The hub has around 1600km / 1000mi of loaded touring use. They supposedly come adjusted a little tight from the factory, so I had readjusted them myself when new, but have never opened them. The grease is in good shape and there is no evidence water has ever entered the shell; however the factory grease is pretty sparse.
I'm assuming most people are using these with the factory grease for the duration of the hub's lifespan. How much distance are you getting out of these hubs? I've attached photos. How does it look? Should these hubs be repacked when new?
I have a few thousand miles on my hub, and the only thing I've done is adjust the cones when it was new.
If you have the hub opened up, and you don't think there's enough grease, add some.