There are variety of material made for the pen casing including plastic, metal, rubber and recycled material. Below is the diagram of a typical pen construction in promotional industry.
The shelf life of the ink is greatly depends on the quality of the solvents and chemical made for the ink. In general the ink can last for 3 years with proper storage. The usage life of the ink between the normal ballpen pen, roller ballpen and fountain pen is greatly different. Below is the rough estimation based on the nip ballpen size.
Normal Ballpen – 1,000-1,500m (1.0mm), 1,500-2,000m (0.7mm)
Roller Ballpen – 500m (0.5mm), 1,000m (0.7mm)
Gel ink pen – 500m (1.0mm)
Please refer this post if you would like to know the main differences between normal ballpen, roller ballpen , gel ink pen and fountain pen.