When is it appropriate to describe research as "recent"?

Good question. The semantics of the word "recent", in general, and in academic writing, in particular, is not clearly defined (that is, fuzzy), which makes its practical use quite tricky, as evidenced by your question.

While @vonbrand's answer offers some valuable insights, such as considering the fluidity of a particular scientific field or domain, I would suggest a more practical solution to this problem, as follows. Consider literature that you reference in a particular paper. What is the temporal range of the sources? I think that this aspect could guide you in to where the word "recent" is appropriate and where not so much.

For example, if you cite sources from the current century as well as 1930s, then a paper from 2010 should be considered recent, but not one from 1950. If, on the other hand, your temporal range of references is rather narrow, say, recent 20 years, then you should refer to as "recent" for sources that are from approximately last 4-5 years. You can come up with your own rule of thumb (10-20% of the total range sounds pretty reasonable). The most important aspect would be not the actual value (for the rule of thumb), but rather your consistency in applying it throughout the paper.

It depends on the area. If you are talking about slow moving areas, "recent" could be a decade ago; for something that moves fast, what was published last year is old hat.

Perhaps the easiest way out is to be more specific, "a study three years back..." (besides, the study might be several years back, or be a decade long study, but the journal issue just came out, so the publication date isn't necessarily telling).

As previously mentioned, the meaning of 'recent' depends on the topic of study. What is considered recent in mathematics may not be considered recent enough for computer science. My computer science professors have generally stuck with anything five years old as being the 'oldest' an article can be. Two to three years is generally better, especially in the tech field as things progress at a much higher rate. A good thing to look out for is when an article might pass the 5 year mark, someone will most likely have adapted the methodology or research findings in a more recent article. Best of luck!