I built a company as a "part-time founder". I think it's quite do-able and really forces you to focus your time. Parkinson's Law states that tasks tend to expend to fill to time allotted for them... That's true with Founders as well.
That said, I think your day job needs to be fairly light for this to work. At the time, I was doing light consulting after having sold my first company.
I'm currently working on another little startup idea (http://www.rescuetime.com) but it's been moving a LOT slower, as my day job is something I invest a lot more time and energy into.
Of course, it very much depends on the scope of the problem you are trying to solve. You're never going to build the next salesforce.com as a PT founder. ;-)
I wish part time work was easier to find. I would much rather work 20 hours a week for someone else than 40 hours a week. It'd give me a lot more time to work on my own projects.
You can only be so productive anyway. Just because you spend 8 or 9 hours a day at work doesn't mean you're working the entire time you're there. More like 1/3 of the time is actually spent thinking and working. It's not even that you're being lazy, it's just you burn out if you work too hard and too often.
The strange thing is, nobody who's in a position to hire understands this. And if you do find part-time tech work, you're most likely going to be paid LESS per hour than you would if you were doing the same thing full-time.
Even stranger: people who work in manual jobs and even in healthcare, do actually work the full 8 or 9 hours. So, maybe we ARE goofing off (me included).
short answer: part time sucks. long answer: part time sucks. i did a variant of this (school for part of it, work for another) for 3 years with my first company, and it was frustrating because you miss opportunity after opportunity because you move so slowly, and everything just turns out mediocre.
so if you have to do your startup with a 40-50hr/wk job, do it as briefly as possible until you can prove to yourself the worth of the idea and then make the jump. and this may sound strange, but get yourself in decent physical shape so that you have more stamina and aren't exhausted after a normal 8-9 hour work day. in my case i had way more energy/focus if i was running/lifting regularly (and frittered around/needed more downtime if i wasn't.)
i'm going full time on my idea at the end of this week and have a lot more momentum because 1) other people will be working on it with me and 2) i will be able to focus 100% and do things like pull all nighters when necessary and basically not have any other constraints in the way of getting things done.
That only depends. Some people are good only 3 hours a day, but you best believe that they will outdo most programmers who put in 8 hours/day.
Nevertheless I would advise you to quit your job and focus full time on your startup.
Do you think about your startup at work, and then rush home to implement, knowing you have a limited amount of hours? A lot of startups have begun this way.
I had a unique idea that I implemented in my part time. The site was featured on Techcrunch a few years back and quickly started making a lot of money. Within a few months a handful of VC funded competitors sprung up. They were able to iterate on the concept very quickly and completely destroyed me within the year by developing a significantly better product then I did.
The moral of the story: Working on your startup part-time is a great way to mitigate risk while you build out your first version. However, if you see some traction quit you day-job and start executing relentlessly.
I think it's almost always a mistake to hope you can get your startup off the ground while working part-time. Usually people will only do the extremely hard (and icky) work necessary when they're presented with a sink or swim scenario.
"...Most founders of failed startups don't quit their day jobs, and most founders of successful ones do. If startup failure were a disease, the CDC would be issuing bulletins warning people to avoid day jobs." -- The 18 Mistakes That Kill Startups
I'm onto my second startup. My first one is (just) profitable and my second is showing very promising signs.
I put all the profits back into my businesses, so I work three days a week to pay my personal bills (and temporary business expenses). Fortunately, my job lets me go in whenever I want, which helps me juggle my time.
I think it works pretty well. It's meant I've never had to take any outside investment and yes, working my day job gives me time to reflect and come up with new ideas. It undoubtedly makes me more productive.
I currently have a job like that but it is not too much of an advantage. One still has to put in 40 hours of productivity and you get tired at the end of the day like any other.
You actually work a day job part-time and work on your startup full-time. That's the reverse, which seems a lot better to me. I still think the ideal situation is over-time on your startup alone.
That said, I think your day job needs to be fairly light for this to work. At the time, I was doing light consulting after having sold my first company.
I'm currently working on another little startup idea (http://www.rescuetime.com) but it's been moving a LOT slower, as my day job is something I invest a lot more time and energy into.
Of course, it very much depends on the scope of the problem you are trying to solve. You're never going to build the next salesforce.com as a PT founder. ;-)