need infos about Ctype visa of schengen

[mockingbirddd]

Hi...
i have got schengen visa C type, and its says that i got it from 29.12.2008. till 17.02.2009., for 51 day, and its MULT entry visa...
i wonder, does the date, from 29.12-17.02. is the period in which i can entry into schengen country's or i can enter after 17.02.2009. until i use all 51 day of visa, coz i have used only few days of that 51 day!!!

thx in advance