I've been trying to copy this accent and I have noticed a few things. Newscasters really seem to open their mouths when they speak and open their jaws after most vowels. Is this the key to their accent? How long should it take a native speaker to learn this accent?
What accent are you referring to? In which countries do the newscasters you admire perform their duties? Quite simply, it does not matter where or in which language a newscaster uses his or her skills as long as they do so in a clear and easily understood manner. They would hardly be employed as newscasters in the first place if they did not possess the ability to convey the news to listeners with clarity of speech, even if there is a regional accent involved. Correct enunciation and ease of delivery are primary requirements I would think, whether it is in English, Swahili or any other language, it doesn't matter which.
Here in the UK the BBC at one time insisted on the standard RP (Received Pronunciation) form of English which now is no longer the case. Hardly anyone speaks that way in Britain any more. Listening to old BBC broadcasters is quite funny really, and by today's standards they sound a bit ridiculous and stuffy. Nowadays newscasters can and do use regional accents delivering the news but in a clear and concise manner which is understood by everyone.
In the states, local newscasters almost never have any discernable accent (except for the weather and sports people). They speak a kind of American "Broadcast" English that is mostly devoid of any regionalisms.
This tends to be somewhat different at a national level, since the network anchors for major networks are often more respected for their journalism than their image. Thus, Dan Rather still has a slightly Southern twang and Peter Jennings still sounds very Canadian.
I want to developed an American "Broadcast" English TYPE OF accent.
Have been listening to WWII BBC broadcasts on the BBC's "On This Day" website. The newsreaders' RP accent is really very different from what one usually hears on the radio these days. Funny how the old newsreaders pronounced "very", "really", etc.
Any kind of American "Broadcast" English would, by definition, be a discernable accent. As with any acent you're trying to copy, it's going to take practice.
As Damain points out there exists not such thing as a "a newscaster accent". I've been watching the BBC too and have noticed a wide range of accents.
I've noticed on the BBC that the people sitting at the desks have much more RP of accents on average than the reporters do. I enjoy trying to guess where the reporters are from in the UK to amuse myself.