From January, 2005 through May, 2006, I worked on StyleFeeder on the side - while I had a pretty demanding day job, mind you - as I built up the basic business... until I had invested so much time and effort into it that I was maxed out and needed to find a way to work on it full time with the help of others.
I don't see any plausible way that I could have made it into anything significant while at the same time working a day job. I think some people can do it with some businesses, but I think it would have been impossible in my case.
But the bootstrapping phase, yes, I think you can do that while working a day job. That's very common.
More details here:
http://www.whirlycott.com/phil/2011/08/05/stylefeeder-histor...