The Head of Visa Policy of the European Commission explains in his letter possible periods of stay for multiple entry visa holders
It is a crucial information for the travellers, that holders of a multiple entry visa with a validity of between 6 months and 5 years are entitled to a stay of 90 days/180 days calculated from the date of first entry. This is clarified in the operational instructions (Commission Decision C(2010) 1920 final of 19.3.2010 amended by Commission Decision C(2011) 5501 final of 4.8.2011) Part II, point 9.1.1.3.
The whole answer can be found here
