How much time on a sketch is "too much"?

I've started to feel like I've been spending too much time on my sketches for my Sketch-a-Day challenge. So much time in fact that they exceed what a sketch is in my mind. I've been producing more and more near completed works lately in lieu of sketches. Don't get me wrong, I'm THRILLED with what I've been posting, I just should be spending less time with less rendering. A sketch should be loose and fun! It should be a rough interpretation of an idea for later production. I think what I need to do is start to recognize at what point the lines on the paper stop being a sketch and start being an illustration. Once I see that I can snap a picture, call the sketch done, and if I desire THEN continue on to render the image. To start with I'm going to impose a 30 minute time limit for my sketches to try and regulate myself. Let's see how this goes for a bit!