The good news is much more than you think. The marginal value of the last hour put into a business idea is usually much less than the first. The world is full of ideas that can be executed with 10 to 20 hours per week, let alone 40. The number of projects that are truly impossible unless you put in 80 or 120 hours per week are vanishingly small by comparison.
Just read this article. It’s by David Heinemeier Hansson, it’s great.