According to this, 1968. Then again, is it 1969?
"‘FOR QUEEN AND COMMONWEALTH’
These words lay under the royal arms on the front page of every issue of London’s Daily Mail until late 1968, succeeding the motto ‘For King and Empire’ which adorned the masthead during the Second World War.1
It could be argued that such a statement was an anachronism rather than a credo, much like royal arms and titles which continued to claim France for
centuries after the end of English rule. Yet the updating of the banner to ‘For King and
Commonwealth’ in the early 1950s, and its ultimate removal on the eve of the 1969 Prime Ministers’ Conference, suggested that the phrase was not simply a relic."