"There is no specific time period after which you need to provide refresher training or formal assessment. However, you may decide that automatic refresher training or a retest after a set period (for example 3-5 years) is the best way to make sure employees remain competent."

HSE site

We tend to base it on experience, accidents, reported near misses etc. Although we max out at 5 years. Most places use 3 years in my experience, mainly because that is what is pushed by the training companies. If you can safely push it back it can be a significant saving, that can be better used elsewhere.

most stuff like this is as a result of trying to avoid prosecution...considering that NHS staff on bad backs alone stand at around 16% which is actually a huge number...never mind, to my point.  there is no legal requirement to have certified training or indeed medical checks for that matter...PUWER says adequate training so you get to decide...on the training and the frequency..I know of some companies who use 5 years and I kn ow some who use 3...