I like Aaron's post on this topic- http://www.aaronsw.com/weblog/productivity

I currently work at 6 startups, 12 hour shifts, over 24 days per month. The 2 main startups I spend 48 hours per week at. The other 4 take up 36 hours per week. One is very profitable and I use it to fund the others. However, I don't think this is a smart strategy at all. I'm just wasting money but having fun intellectual pursuits.