I've maintained two Nexus 8-speed hubs for about 15 years by the "oil immersion" method. Once a year has worked for me; this probably represents around 1,500 miles.
The oil is a synthetic gear oil ("Royal Purple" 75w-140), which is a lot less expensive than the "official" Shimano oil.
I made a container for the immersion process to minimize the amount of oil needed.
The bearings are re-lubed with "Nexus grease" before reassembly.
I did not de-grease these hubs; the original grease has gradually disappeared over the years.
Another resource for servicing your hub is Rat City Bikes (Aaron's Bicycle Repair).


A plastic syringe attached (epoxy) to the bottom of an ice cream jar accommodates the axle.



This can is the right size to serve as a stand for the immersion vessel. the oil level shown is about right; it rises to the top when the hub is inserted.