Current greased IGHs do require regreasing at 3000 to 5000 mile intervals normally speaking. Some owners shift to oiling through the shifter hole in the axle with good results. Shimano sells a oiling kit which requires partial hub disassembly if you follow their published procedures. Older IGH units with an oiling port in the hub normally required a few drops of oil every week to month or so depending on mileage ridden.

Other than lubrication maintenance is minimal normally speaking. Very occasional shift cable adjustment may be required, particularly after wheel removal and reinstallation.