As far I know, the period for which a visa is issued is strictly dependent on the reason of issue. The employer shall issue a declaration of intention to work for a for a specific period of time, and this period is a measure of how long a work visa is valid. For example if it's issued for 5 months, the validity period of work visa will be also 5 months.
Individual Country Visa (D) A "D" visa allows a foreigner to enter a single country and stay within its borders for up to one year. The validity of such a visa depends on the purpose of travel.