How long will my tattoo take? 

It's a complicated question, and it's tough to give an estimate until the drawing is done and sized on the body. 

Factors on how long a tattoo takes include the size, the level of detail, the size of the line, and the placement on the body.

Thicker lines tend to go faster than finer lines. 
More detail/texture/stippling takes longer. 
Solid black and starry skies take longer than just lines.

What does your budget tell me? 
If your budget is larger in relation to the size of your design, we can use finer lines, more detail, stippling, solid black, etc. 
If you are hoping for a piece with a smaller budget, I'll gravitate towards thicker lines, less detail, less stippling/black fill. 

Some extremely basic estimates:
Forearm full single side: 2-3 hours
Forearm complete wrap: 4-6 hours
Upper arm half sleeve: 3-5 hours
Full sleeve: 6-8 hours+
Inner upper arm
: 1.5-2 hours
Back of upper arm: 1.5-3 hours
Shoulder cap: 3 hours
Upper back: 2-4 hours
Large back: 3-6+ hours
Chest: 4-6+ hours
Sternum: 2-4 hours

Thigh: 3-5 hours
Calf/shin: 2-4 hours