I agree with the first part of Uncle Stevie's advice, although in my case I wait at least six months. Sometimes I wait as long as a year, or even more. The further away it gets, the more objective I tend to be.

Regarding starting a second fiction project, the answer to that is yes and no. I slightly take issue with Uncle Stevie here, as I think that can lead to burnout. That said, I have gone from novel to novel before but only when the voices in my head for the next novel are screaming. In your case, I'd prescribe a few weeks of short fiction pieces where the stakes are lower. If you fail with a short story, it isn't the end of the world (I've had more than my fair share I deemed not up to scratch). :)