I would not let any green coolant go more than 2 years. best is every year. Even this new long life stuff should not stay in longer than 2 years(IMHO). Additives that prevent electrolsis do not last as long as these companies represent. If you want to have the coolant last longer, install a coolant filter with additive packages to protect against electrolsis and other corrision(just like a diesel).
It has been a few years since I have looked into this stuff, but gates had some great info on coolant and such. We here in the US have a open system which allows oxygen to get into the coolant system. With this indtruduction of O2 into the system the fluid degrades. Over in europe, the systems are closed and do not allow the fresh introduction of O2 into the system, so the coolant can last longer.
That was from some studies a few years back.