"Long timescales" refers to a long period or duration of time. It means that something happens or changes over a considerable amount of time, usually spanning years, decades, or even centuries. Full definition
We had written about it the year before, in one of the first papers on the divergence problem — I think other groups had actually called it the divergence problem — and, since then, we have been working with other tree ring data trying to improve the way we process the data to try and make sure we keep as much of the low frequency information on longer timescales in the trees because you have to standardise trees in a certain way to produce temperature reconstructions. (climateaudit.org)
But a lot more research will have to be done before we can be sure these positive effects persist and outweigh any negative ones over the very long timescales involved in storing radioactive waste. (newscientist.com)
When the project started, we mostly ran global models looking at long timescales of decades, but more recently we have been running regional models looking at much shorter time periods of months to a few years as part of our weather@home series of experiments. (climateprediction.net)