No, there is no time constraint. But an editorial  image could lose value depending on the subject. For example, I have an editorial image of a band busking on Grafton street in Dublin, that image keeps selling even after two years. But my editorial images of referendum campaign posters from two years ago dont sell any longer.

I tend to play on the safe side, as do several of the agencies. (iS is very safe, SS apparently not so much). 
Find out when copyright expires in the country concerned. E.g. in the EU, copyright continues until 31st December 70 years after the death of the artist. However, France has an extension for war service, if relevant.
Here is Wikipedia's account of many countries' copyright expiry:
http://en.wikipedia.org/wiki/List_of_countries%27_copyright_lengths
You could always double check to be on the safe side.
The agencies will normally build a very wide 'fence around the law', for example in the UK, a 2D representation of a 3D object does not break copyright, but you'll normally find the agencies will regard anything within the copyright period as a no-go area.
Very generic objects are different, though.
It's complicated and confusing, and the agencies don't all operate the same policy. Still, I still advise you to err on the safe side: who's got time for a court case?