Hi Latchy

It varies, in my experience. A lot will depend on how often you are doing manual handling tasks - if you're using your MH skills all the time a reasonable time for refresher might be every 3 years, more frequently if your not using them quite as often. I don't know about course lengths though!

There is no legal standard around this.
In fact there is no regulation requiring training (although of course your assessment may identify this as a control measure.)
The main reason training organisations put a 'valid for' date on such a certificate is to get more business.

If people are practicing MHOs, and being properly monitored (supervisor watching from time to time) to confirm that good techniques are being used, why do they need 'refreshing'?

If people need training, they need training, regardless of a time period. If they are doing OK, not needed.

I am no more likely to forget how to lift than I am how to sit.