totally depends on the image. most of what i'm asked for, i can draw pretty quickly, 20-40 minutes. a lot of my "custom" pieces are done as walk-ins, so i actually draw a lot of my stuff pretty quick and hardly ever draw at home. i actually spend more time on drawings for regular customers (meaning not seeking me out specifically) since those tattoos tend to be more outside my comfort zone, so it takes me a little longer to visualize it, then work it out on paper.
i prefer drawing at the shop, since there's a copier and a ton of reference, and i do reference EVERYTHING. but i do like painting at home, after the woman and child have gone to bed.