"Don't confuse efforts with results"
I heard this at school over the past year. I don't recall who told it to me, but they where talking about new sculptors that spend lots of time on their art and it ends up looking bad. Instead of the student working on the sculpture a little longer to make it better, they use the excuse that they spent a lot of time on it. Just because you spend a bunch of time on something doesn't meant it will look good. It doesn't matter how long it took, it matters how it turns out.
I remind myself of this when I am burnt out on a sculpture that is 99% done. I ask myself if I would be embarrassed to have my name on it at this stage. Would I be uncomfortable presenting it to a potential customer? For me the rewarding part of making the art is finishing out what I set to do. Some times I take a few days break before I tackle the finishing touches.