It kind of depends on the complexity and size of the site. You want to make sure you can have it done in a reasonable timeframe.

I'm doing all of the development on my site http://collegeacb.com but its a really simple concept (basically just a forum). One thing I have found really helpful though is having a cheerleader. Nominate someone to keep you focused (even if they don't do anything else) so that you don't give up half way through.