The way the IFC, other multilateral institutions, and indeed the CDC in the U.K. work, there is a private rate of return, which is the financial rate of return. Then they try to compute something called the “social rate of return”, which is an attempt to capture those wider gains to society from investment.
Usually, although this is not in every case, a good long-term benchmark is to say that the social rate of return should be at least twice as high as the private rate of return. The private rate of return should be about twice as high as the long-term real rate of interest in a country. If you achieve 5% to 7% private rates of return and 12% to 15% social rates of return, you're doing very well. In fact, you might be beating the international average.
Your point about the long-term nature is an important one. If you choose to go into something like green technology, which is highly risky, it may take time to achieve that rate of return, and you have to be tolerant of a lot of failure, just as we have in this country with start-ups, to find those one or two successful firms.