30 mins.

I think that is in the HSE guidance somewhere. Seems a long time to me.

And as I am sure you know mike, but some visitors to this site may not, the duration is only one factor - you may have a 5 min task that would not be undertaken from a ladder.

I am reliably informed the 30 minute 'rule' as per HSE guidance (Working on ladders and step ladders) came about when the HSE monitored some painting of a window frame on a ladder and subsequently came up with 30 minutes, the time it took to paint the frame. Notwithstanding that, I believe there was a case where a project wanted to use the 30 minute rule and then move the ladder a few metres to carry on with the work for a block of flats. The HSE argued that this method of work was not in keeping with the spirit of the guidance and scaffolding would have to be provided.

Even though the HSE state a maximum of 30 minutes (and light duties) for working on a ladder, i suggest anyone who has to stand on the rung of a ladder for 30 minutes should be able to find a better and more comfortable and suitable platform for their feet to stand on.
That's my opinion.