According to the 22nd amendment to the US constitution An American president is to served for 10 years and that's the limit.The individual can be elected into office for two full terms which is 8 years but if such person was elected through the order of succession then the person is entitled to an additional 2 years which brings it to 10 years maximum.
An American president can:t go in a third time.This constitution was outlined since Feb, 27,1951, it was concluded all individuals going in for presidency can be served for just 2 terms
I think this is what most country adapts even my country has same duration for any elected President. So one should try to accomplished all that he or she can within that time frame. .