It depends on the piece, but generally these ones fall into the 2-4 hour range. I usually chip away at them, whenever I have some free drawing time. More often than not, I find myself working on them during my lunch break and in the middle of the night.
This array of Pokemon art especially has really helped wind me down, after working on other folks' art requests all day. It's a PokéRelease~
And to those who commented about making money off of art - Thanks, and I am. ^^ I've been a free-lance illustrator for nearly 4 years now. (Hey! That makes me sound old ;____;!!)
Thanks again for all the comments. ^^