in living memory

[Elmek]

how long is it? someone told me it was only 20 years but surely its longer than that, any suggestions?

[Muffet]

At a guess, if you said that some period of unusual weather had been the wettest/coldest/hottest in living memory, I would take it to mean the one longest ago that someone alive could still remember. Ditto in relation to any other person or event.

[jno]

just so, though I don't suppose anyone using the phrase actually checks to see if anyone remembers something. But in general it probably suggests somewhere round 80 years, since that's as far back as anyone's likely to remember. (Say 1925.)

[jno]

... having said that, I suppose the first world war is still in living memory - not many survivors left, but there will be people who were too young to fight but who still remember it.

[Quizmonster]

These things obviously change regularly, but I believe Britain's oldest woman is Annie Knight at 111 and our oldest man - who is also the oldest surviving veteran of World War I - is Henry Allingham at 110.
Not sure about the lady, but Mr Allingham is still completely compos mentis and recalls the First World War vividly. Indeed, he recently went to Europe to meet Germany's oldest survivor of that conflict. I see no reason to believe, therefore, that he cannot recall things from before that time, having been born in 1896.
However, I'd say the phrase 'in living memory' would require more than just one or two people to be involved in remembering. Nevertheless, I'd be tempted to say it stretches to a century.

[heathfield]

We are told in the Bible (Leviticus, Revelations) that the 50th year should be reserved as a jubilee of remembrance of events. Maybe something to do with it?