I don't think there's any real rule of thumb, it all depends on what the intent is with the work. You may not be doing it as a narrative tale but as an alternate history text or even a pure date:event format there is some intent.
To take some examples, a lot of those President ATL McATLface back in the Old Country don't last much beyond the tenure of the President in question. Why would they have to? The story is about President McATLface, and their story is ended. Similarly,
@Blackadder Mk2's
The Crowned Prime Minister and
With Nowhere Else To Turn don't go beyond the premiership of Douglas-Home and Thatcher, respectively, but that's because those are the premierships that they concern themselves with. It doesn't mean you can't get over some glimpses of what has become after these Presidents or Prime Ministers have left office through suggestion in the text.
To give a counter example, I enjoy many of those 18/19C US TLs that mix narrative and text entries - so much so I foolishly tried to right one, that stupid boy really shot himself in the foot there! However, I find that without a clear end goal point or one too far in the future these can quickly become bogged down and lose steam as they go on, ironically often as steam power really takes off.
A very verbose way to say, it depends.