I trained as an historian to doctoral level, but spent six years working professionally as a journalist and have continued to freelance since that time, so I hope that I'm in a decent position to draw some lines for you here. I should stress that these lines are not hard-and-fast, but I do think that the core differences between history and journalism are about a lot more than just the age of the topic.

First, we need to bear in mind that there are different specialisms in journalism as well as in history. If you are a news journalist, your job is to report on events and developments that are important to your audience as promptly and as completely as possible. "Important" can mean different things. You might be a newspaper journalist reporting on world events that won't impact directly on your readers, but which, nonetheless, they feel it is important to know about because they change the shape of the world they live in, in a minor or a major way. But you might also be reporting in the trade press to a professional audience that needs to know about government policy, or new product launches, or something that's happening to competitors.

Either way, the key point is that news journalism is immediate. You're conveying information that is significant or meaningful now, but will be less so the next time your publication actually publishes. Again, time is quite a flexible thing here - a daily paper is, famously, just tomorrow's fish-and-chips wrapping (at least that's how we express it here in the UK), but I've worked for weekly and monthly and even quarterly trade press titles that still had a major commitment to reporting news. So the imperative is to cover the news as completely, and in as balanced a way as you can, against a deadline. If you miss that deadline, you might as well not have bothered in the first place. Your competitors will be covering the story, so you are going to lose readers if you cannot keep pace with them. Of course the same applies if you cannot cover the news as well or as accurately - if you are constantly being scooped by rivals, you are also going to lose readers. The point is that there is a strong commercial imperative (which eventually translates for individual journalists into strong career imperatives, because you need to keep your job and you are being assessed on your ability to meet readers' needs as well as, or better than, your competitors) to be timely, which is ultimately more important than any other consideration.

This means that journalists accept from the start that the stories they write are only going to be "complete" in the most comparative sense. There will be things your sources won't tell you, and contexts you won't be aware of at the time you write. You publish anyway, and everybody involved, from your bosses to your readers, understands and accepts that you are confronted with problems and limitations that can't be overcome in the time available.

Of course there are also journalists who write features, or commentary, who aren't confronted with precisely the same problems as "hard news" journalists. It's their job to contextualise, join dots, and even speculate in ways that news journalists aren't usually expected to do. But in pretty much every case, these writers still face rather similar limitations in terms of the completeness of the information they can access, and their reporting is assessed and judged in similar ways. No one really expects them to be complete, and in some contexts they wouldn't even be expected to be neutral or unbiased. If you're writing commentary for a conservative newspaper, you're usually going to be expected to write from a broadly conservative perspective.

Historians do face some of the same constraints that journalists do, which is why I say the lines between journalism and history are not absolutely hard and fast. If you're an historian, you don't expect to have access to every piece of information you might need; there are going to be gaps in the records you consult; sources still "lie" or seek to portray matters in a way that benefits the person or the group that created them, even if they are hundreds or thousands of years old. And historians themselves may still write in a way that is openly biased - they may approach history from a consciously Marxist perspective, for instance - and will be impacted by semi-conscious or unconscious biases as well. There really is no such thing as absolutely "neutral" history.

Nonetheless, if you are writing as an historian, you are supposed to do two things that are not expected of journalists. First, you are expected to have sought out, and read, and thought about, everything that matters to the topic you are interested in. Now, "everything" is a very moveable feast. If you are writing on very modern periods, a lot of stuff is still going to be restricted, and if you are looking further into the past there are going to be other sorts of problems to confront. It's possible to have pretty much literally read everything there is to read, if you are writing about eighth century Saxon England – though you'll certainly also be aware of the yawning gaps where you want to have something to read, and absolutely nothing exists. If you are writing about the French Revolution, it's simply impossible to have read "everything" - not only in terms of secondary sources, but primary sources too. And nowadays there's a lot of pressure on historians, and every other sort of academic, to publish regularly, and maintain some sort of publishing "schedule", if they want to get a job and win promotion.

So it's accepted, by your colleagues and your readers, that you are going to have to be selective. But, nonetheless, the expectation is that you are still going to have assembled your evidence in a way that's quite different to the way journalists work. You're going to be expected to have read representatively and you are also going to have to show, if required, that you have read as completely as is reasonable to expect in the circumstances – depending, that is, on where you are specialising. That includes reading and taking account of the findings and, just as importantly, the opinions of other historians, including those you have fundamental disagreements with - the latter being something that journalists, broadly speaking, rather rarely do. In addition, you are expected to contextualise. That may mean being aware of the historiographical debates that impact on your topic, or it may mean placing a very specialist study in a broader context of time or place.

Journalists are expected to do most of these things as well, to a greater or lesser extent, I know, but I do think that it is here that the critical difference between history and journalism lie. For journalists, it's a matter of doing what you can to tell the whole story in the available time. For historians, it's a matter of taking whatever time is necessary to tell the whole story in its fullest context. And that's because, ultimately, it's an historian's job to render judgement on some complete problem, whereas it's a journalist's job to provide pieces of the jigsaw that combine with other pieces (published and yet to be published) to let the reader assemble a picture on which they will render judgement, and perhaps take action. And all the commentary and features journalism that runs alongside the news journalism has a similar purpose.

In other words, while both historians and journalists are in the business of persuasion, historians try to persuade by presenting findings that have already been both contextualised and assembled into a reasoned case, whereas journalists (who may try to be, and think of themselves as being, neutral, or may be just as keen to persuade as any historian) are more usually presenting information, and contexts, that are not fully contextualised, and which somebody else (the reader) is often going to assemble into a reasoned case.

So, to answer your question directly, the time element you are interested in doesn't operate in quite the way you are suggesting. It's not so much about how long it's been since the topic under consideration was current, or since the events concerned took place. It's much more about how much time the journalists, or the historian, will be expected to invest in researching and writing about that topic or problem.