You could go on halving the sentence forever by your system, Gemini21!
As I said above, someone sentenced to 12 months imprisonment is normally released after 6 months. The prisoner can apply to have 'police days' (where he had to attend a police station) deducted from his sentence. If he's due for release on a Saturday or Sunday, he'll actually be released on the Friday. Other than that (unless he gets onto a tagging scheme or he becomes terminally ill) he won't get out any earlier.
Chris