I have a couple of tattoos, but I'd like a partial sleeve. Maybe a half-sleeve or quarter-sleeve on my upper arm. 
How long does it take to get a half-sleeve done?
Well, there's a difference between an arm covered in tattoos and a sleeve tattoo. A sleeve has a unified theme and may take many sessions to complete. 
How painful is it? I have never gotten one.
Certain parts of the arm hurt more than others, the inner arm and elbow areas for instance. The pain comes from using needles to insert dyes and pigments into the skin. 
Do the colors in the tattoo fade over time? I seem to always see people with faded and dull tattoos.
down through the epidermis and upper dermis, in both of which the presence of foreign material activates the immune system's phagocytes to engulf the pigment particles. As healing proceeds, the damaged epidermis flakes away (eliminating surface pigment) while deeper in the skin granulation tissue forms, which is later converted to connective tissue by collagen growth. This mends the upper dermis, where pigment remains trapped within fibroblasts, ultimately concentrating in a layer just below the dermis/epidermis boundary. Its presence there is stable, but in the long term
Sleeve tattoo
A sleeve tattoo (or tattoo sleeve) is a large tattoo, or a collection of smaller tattoos, that has a unified theme, that covers most or all of a person's arm, usually from shoulder to wrist.
There is a difference between an arm covered in tattoos and a sleeve tattoo. A sleeve tattoo has a unified theme, whereas an arm covered in tattoos may have many tattoos of different styles that does not have an overall unity.
The term "sleeve" is a reference