A 'not guilty' plea can affect the sentence passed by a court. (i.e. anyone who pleads 'not guilty', and is then convicted, loses the 'discount' that an early 'guilty' plea would receive). However, once the sentence is passed, the plea becomes irrelevant.
Anyone sentenced to 9 months imprisonment would normally be released after 4� months. Any time spent in prison while on remand would be deducted from that period, as would any 'police days' when the offender was required to attend a police station. Anyone who's due for release on a Saturday or Sunday would normally be released on the preceding Friday.
Some offenders are also eligible for electronic tagging, which might see them being released from prison rather earlier.
Chris