... it seems to me, there's no SET format applicable to the timespans of Schengen Visas; and their duration would appear to be at the discretion of the individual European Embassies issuing them.
Bit daft,really, to increase the validity of multiple entry visas from the [?normal] 6 months, to between 2 and *4 yrs ...
... kinda defeats the whole object of the exercise since, by *then, MOST entrants will have ALREADY qualified for British Citizenship.