as per the IS codes the ultimate criteria for acceptance of cement is compressive strength test result at the age of 3, 7 and 28 days.
So cement do achieve its desired 100% strength at the age of 28 days.
Cement keeps process of hydration for a very long time, till it gates hydrated completely and have water for reactions.
cement strength gain can be go beyond year when it may show a strength from 110 to 130% of what it showed in 28 days.