Regretably I can't give you a specific number of years because copyright protection is related to each author of the newspaper articles etc. The protection lasts for the lifetime of the author plus 70 years after his/her death. So you can see that in the case of a news item by a young reporter, the length of the copyright could be as much as 130 or 140 years, based on an individual living to 80 or 90 years of age.
In the case of reporters etc employed by the newspaper, the copyright will actually be owned by the employer, although the method of calculating the length of the term of protection is still based on the author's lifetime.
Years ago few people worried about copyright in relation to old newspapers, but in more recent times when the press have faced economic difficulties, they have sought to monetise their archives as an additional income stream.
There are some solutions. If you only need to quote relatively short amounts you can do so using an exception to copyright law known as fair dealing
. As the name implies, as long as your use is proportionate and you copy no more than is absolutely necessary, you can use the quotation exception. However this wouldn't allow you to copy a complete article*, but you could re-tell the story in your own words, and only use the odd quotation, for example any reported speech, in order to give your write-up a more authoritative feel. And it is very important to provide a credit to your sources, but I imagine you do this anyway when using the full articles.
Secondly you could approach the newspaper concerned for permission to copy old articles. Depending on the paper this may or may not involve a payment. Of course this becomes more difficult if the newspaper no longer exists. You may need to do some research to find out if a particular title was bought up by another company and now forms part of the intellectual property of a current day newspaper publisher. Your local archives may be able to assist with this.
Thirdly, if you really want to do this on larger scale, you may be able to get a licence from the Newspaper Licensing Agency.
You would need to check their online list of titles they represent to see if the ones you are interested in appear there. However bear in mind that the NLA is largely there to licence modern day newspaper content, so it is probably unlikely they will represent the older newspapers you are interested in, unless it is one of the national dailies or well established local newspaper.
If the newspaper(s) you are currently using as the source of your stories has been digitized as part of the ongoing project
by the British Library and its commercial partner Findmypast, you will need to approach them about licensing, as the situation varies depending on the title concerned.
And finally, if your newspaper no longer exists and you haven't been able to trace any present day owner of the copyright, and the British Newspaper Archive doesn't include your title, you may be able to obtain an orphan works licence
to use specific articles. The down side of this is that you would need to submit an application for each individual article (although up to 30 applications can be grouped together to save time and effort). You would need to provide evidence that you had conducted a diligent search for the owner of the copyright before submitting your application.
* the situation with photographs is slightly different, because it not clear if a photograph can be 'quoted' since that would inevitably mean copying the entire image. However since your project is not for a commercial purpose, it may be possible to take advantage of another of the fair dealing exceptions, this time the one for the purposes of research and private study
. Once again this falls within the umbrella term of fair dealing and so a credit is needed for your sources.