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you are entitled one month "good conduct credits" for each year - so automatically your time is cut by one month
additonally, you can earn up to 7 1/2 days a month for work assignments and/or participation in programs. so that would knock off about 8 days a month - and so it looks like about 12 months - if you get all the good conduct and other credits you can earn.
keep in mind, that credits can be taken away for bad behavior and then that adds the time back onto your sentence.