To create one piece?
Not long, really. I usually get 6-7 done in a day, and that leaves time for real life the rest of the week. Haha
Some of the more complex designs take longer, of course (like the maple syrup one or the Dalek, to name a couple of recent pieces).
But when I do photo manipulations like the bigfoot one for @papa-pepper's recent contest, that type of thing takes a bit longer.