Assuming we're talking about comic pages:
Generally I'm unwilling to sit for more than three hours to draw a single page. For The L this means I finished one page (54, due out 19 March 2011) in a single sitting. Most take two or three days to complete, from writing to final drawing. From there I scan it and edit it on the computer, which takes several hours in itself (maybe six or eight, sometimes more). This is usually spread out over a couple of days too. It takes about six days from writing to completed page, which is why I update every week. Some early pages took much longer. Page 22 has the distinction of the longest time from start to finish of any page I have ever done, 564 days. It was the map's fault.
For Trapped in the Closet, each page is completed in a single day, from writing to completed page, because most of the work is making things black on the computer, which takes hours, but is very easy. I would have to say that for the first time ever I'm completely satisfied with the level of black on each page.
Assuming other works:
It depends. Drawings can take hours, and then I tool around with them for days to months, getting them just right. Videos generally take less than ten hours to complete. I have a game that I've been working on since May 2008 that looks as if it will never be completed. Fully printed out the board will be about six feet long and game play is expected to take days. I build scale models of vehicles, weapons, and people out of cardboard and they take very long. The average person, with eight points of articulation, takes about twelve hours to complete. The ones with ten points of articulation, of which I have made only two, take much, much longer. Every one I make I swear it's the last one because of the living hell of working on something that small, that detailed, over that long a time, often having to redo things multiple times, with that unforgiving a medium, and each time I come back for more like some sort of masochistic architect seeking to populate his world with ever more subjects so as to increase the realism. I've been working on a building for months that I probably won't ever finish. I've been writing a novel since 2006 that is also unending, seeing as how each chapter turns into a seperate book. I'm up to six so far, one of which is finished, with three side projects.
So, are you spending too much time on your art? I suppose if you're like me you could ask the question "am I suffering enough?" and if the answer is "no," then, no, you're not working on it enough. If you're not like me then, firstly, I congratulate you for avoiding that situation, and secondly, you can take great solice from the fact that you are not like me. Set me up as an example of the guy who takes too long to do things and takes on way too many projects at once.
And now the return of the best smiley face of them all, in spades. It's so cute.