It's a piece of string answer.
I voluntarily cocooned myself on the 10th March because I knew from people in France how things were likely to develop, the 10th was the last time I did any shopping.
I was caught in the NHS net on a subsequent trawl.
On 23th April I got the shielding letter specifying 12 weeks from the date of the letter which was 17th April, that gives me an end date of the 10th July.
My original action was solely because of age, I don't have any of the listed conditions, I think the NHS action is because of an acknowledged error in my NHS details that I have tried to get corrected several times, there appears to be multiple routes of promulgation.
I did not register because I had adequate support, which was advice in the letter, I've since been contacted by both the NHS and the LA hub to ask if I needed any help.
I'm monitoring 'new cases' more than fixed release dates.