I think there are two things contributing to the short glow time on the newer diesels. One is that they are direct injection engines rather than the prechamber design used on the OM606 and earlier. The older indirect injection engines lose significant heat through the prechamber, which makes them more difficult to start (and is also why they deliver lower MPG).

As well, glow plug technology seems to have improved. They heat more quickly.

As to whether the glow plugs stay on after starting on the newer diesels, I am not sure. I think the answer is yes, but am not certain.