Sorry to confuse things but i thought it was every 3
As far as I understand any of the 3 year test period cylinders that are in date should have now been tested to a standard allowing them to go into a 5 year schedule - (See HSE-AL-FW2 below) Any such cylinder last tested at 3 years ought to now be out of test which means it would require testing before use and then falls into a 5 year period.
Though as below HSE-AL-FW1 only allows for 3 years, but I understand any would now be time expired.
There is an important point that is implied though - don't rely on the details of the rules etc - the cylinder must have marked on it a test label which shows the life until the next test, and you receive a certificate when cylinders are retested. Hang on to the certificate to support the label.
If a cylinder gets scratched etc or has any work done such as removing the regulator it should be retested.
FIVE YEARS FOR FULL-FIBRE WRAPPED BOTTLES - EN12245 and Pi certified.
THREE YEARS FOR FULL WRAPPED BOTTLES - HSE-AL-FW2 manufactured before July 2003 then five years after retesting for SCI and Luxfer cylinders
THREE YEARS FOR FULL WRAPPED BOTTLES - HSE-AL-FW1
FIVE YEARS FOR HALF WRAP BOTTLES
FIVE YEARS FOR ALUMINUM OR STEEL BOTTLES
http://www.ukpsf.com/paintballlaw.asp#1