I think it depends on how many projects you have going on at once and, of course, their nature.
If you're an artist, your workday generally allows a lot more room for "failure". The number of upcoming shows and events I have lined up have turned my studio into an art factory. There are a multitude of pieces being created at once. Working at this pace, not everything turns out as you'd hoped. I'd say that earlier this week I was working at a 50% success rate. Yesterday I painted over two almost-completed canvases that just didn't seem good enough to let live another day. I did, however, complete a 24" x 30" piece I feel very good about.