Your occupational health adviser should be advising you about this (you do have one ...?), but typically it is repeated after 1 year and thereafter done either annually or every 2 years dependent on risk.

Have a look at Page 112 of HSE Publication L108 (http://www.hse.gov.uk/pubns/priced/l108.pdf) which states the following in Appendix 5 which relates to Audiometry Testing Programmes "The schedule of audiometric testing should include annual tests for the first two years of employment and at three yearly intervals thereafter. More frequent testing may be required if significant changes in hearing level are detected or exposure conditions change, increasing the risk of hearing damage...."