The electricity at works act (EAWR 1989) states a maximum of 5 years. This is assuming that you have a full history of previous test results. 90% of the time most companies don’t know where to find the previous test results, all they are interested in is getting a piece of paper and filing it without reading properly. This leaves the company open to massive litigation and fines for not following EAWR 1989, which says that in order to comply with the requirements inspection should be carried out as per GNC3 in conjunction with BS7671AMD3 (at present this will change when the 18th addition comes out) but it is also up to the inspector to determine the actual re-test period which he/she will from many years of experience be able to assess the state of the existing installation and therefore recommend re-testing at either the 5 year maximum or less depending on how bad it is.
There is a list for various types of property which is on guidance note 3 (gn3), but please be aware that these are the maximum periods between testing allowed, however they may be less.