How long is the Schengen-visa valid for?

In general you receive your first Schengen-visa with a validity covering only the duration of your planned stay in the Schengen-Area (e.g. from November 1 to 12).

For subsequent travel(s), the Embassy can issue Schengen-visa(s) that permit the holder to stay in the Schengen-Area for a certain number of days within a certain period of time (e.g. 30 days between March 1 and May 31).

The maximum duration of stay in the Schengen-Area is always the number of days that are stated in the visa! The duration is counted from the date of the first entry into the Schengen-Area.

A Schengen-Visa with the validity of one year or more and a registered duration of stay of 90 days permits you, however, to stay in the Schengen-Area for a maximum period of 90 days per half-year (in Germany or other Schengen-Countries). Please note that the counting of the duration of stay does not begin with the start of the validity of the visa.

Example: the visa is valid from December 15 to December 14 of the following year; duration of stay: 90 days. You enter Germany on December 23: you may stay in Germany until June 22 of the following year – the maximum duration of 90 days. The second half-year begins on June 23, during with you may again stay in Germany / the Schengen-Area for a maximum period of 90 days (even if the residual term of the visa does not add up to a full half year.)

Please contact the Embassy if you have difficulties in the calculation of the duration of stay, or, if you are in Germany, contact the Aliens Office (“Ausländerbehörde”) that is responsible for the area you are staying in.
See also: “What is a 'Visa with a validity for one year' ('Jahresvisum') or a 'virtual visa with a validity for one year' ('Unechtes Jahresvisum')?”