I think you might be
A Rotating Black Hole
My orientation changes, but contextually I stay the same.
The orientation here is that of the axis of rotation which is relative to the Earth. Obviously will be different for different black holes.
Some prefer me in seconds because I tend to have an easier time opening up.
This is possibly referring to the celestial angle which will be more easily measured in seconds. Because these objects are many lightyears away, they won't tend to move very much in the sky but, with refinement of our measurement to seconds, we can probably more easily detect any changes.
I stay the same, yet I am always becoming.
Black holes do not change, however a rotating black hole gradually loses its rotational energy over time and becomes a Schwarzschild black hole (though there is no difference in appearance).
I sometimes have a lot to say, but my rhetoric is rarely duplicated.
I think this is referring to black hole accretion as observed, for example, from active galactic nuclei. I think the lack of duplication is referring to the range of frequencies observed which should vary over time.
I am often ignored in youth but find an audience in time.
Black holes are difficult to observe and are far away so it does take time before an entity can be classed as observed.
Some come to me for advice, but my dialogue is often finished upon arrival.
Because these objects are so far away the signal is usually completed by the time it reaches Earth.