Mine is slow, I would guess it takes about 1 mile to get some heat, and 2 miles to get good heat, max heat probably takes 5 miles, (town driving). The heated seats however only take about 200 yards, great.
Some suggest it is the efficiency of modern diesels, i.e. lower heat losses. In the old days I would be blanking off the rad, you don't seem to see that any more.