Opportunity costs for an entrepeneur

I had coffee today with another entrepreneur who recently had to wind down his business. We were comparing notes. He had raised 27 million and spent 7 years trying to grow his business and ultimately had to shut it down. I spent 3+ years working on Judy's Book. 
The time spent on a venture that is not picking up steam -- that lacks break out momentum -- has real opportunity cost. As an entrepreneur, there are so MANY ideas, and so little time. Time (and energy) are the limiting resources for an entrepreneur. Reminder to self -- pay attention to my investment of time as a real investment with a real cost.