On a long journey, less than 5%. On short trips in the summer, up to 10% (as the system is worked quite hard on a short journey as there isn't the necessary airflow assisting it). You can cut the above figures when the weather is warm by opening the windows by 1/3 and directing the cold air to footwells for a few minutes (this will force the hot air out of the windows).

Either way, spending a few extra pounds on fuel is more cost-effective than regassing your AC and/or replacing the compressor. The AC system is self-lubricating. Switching it off for three or four months will cause the seals to dry out and starve the compressor of oil.