Waiting for 3 to 8 years? I think that is too much for me. I understood what you are saying but I think a business should look promising already before 3 years. Any business that is not profitable for me in 1 or 2 years, I stop doing the business. I still think that 3 years can be good, but 8 years is too much.
For businesses to become established depends on the nature of the business, but it should be under 3 years.
3-8 years is too much for everyone... 3-6 months is like a grace period, maybe even a year (as you wrote, it all depends on the nature of the business), where business is taking more than it brings! Costs, marketing, and many other things... but after that initial period, you should know if it's promising or not.
If you are losing capital for 3-8 years and waiting for things to get better, something is terribly wrong. People should know when to cut their losses, it's as important as patience.