These daily sketches have been a really great thing. I know they're not always the best, but they've kept my momentum up and helped me learn new things. I give myself up to an hour and a half to finish them although I try to keep it down to an hour or less. It helps keep me from over thinking things and get to the essence of what I want to communicate. As an added bonus (and a counterintuitive one) it's actually sped up my regular work.
If you're an artist of any sort- writing, music, painting, whatever- I recommend challenging yourself in this way. Use your lunch break to try to produce a complete piece. The results may surprise you.