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.
They do, especially with sun exposure. After the initial injection, the pigment disperses down through the dermis layer of the skin to become permanent. 
How do you find a tattoo artist? I would be afraid of just picking someone at random.
Tattoo artist
A tattoo artist (also tattooer or tattooist) is an individual who applies permanent decorative tattoos, often in an established business called a "tattoo shop," "tattoo studio" or '"tattoo parlour." Tattoo artists usually learn their craft via an apprenticeship under a trained and experienced mentor.
Apprenticeships.
A tattoo artist traditionally earns the title by completing an apprenticeship under strict guidelines from an experienced senior tattoo artist. A tattoo apprenticeship can last as long as five years. The apprentice will be
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