A decade was a unit of time equal to ten years. (AUDIO: The Morton Legacy)
The Twelfth Doctor observed that Lord Sutcliffe made the 1814 frost fair "the biggest frost fair in decades." (TV: Thin Ice)
The Brynblaidd cannibals had a tradition of butchering travellers once every decade. (TV: Countrycide)
The Doctor spent "decades" during the latter part of his second incarnation on Darron, learning the psychic techniques of the Mind Monks. (PROSE: The Quantum Archangel)
In 1941, Jack Harkness observed that Rose Tyler was wearing fabrics that would not be around for at least another two decades. (TV: The Empty Child)
The Object was traded, or more accurately, auctioned off, over the course of several decades. (PROSE: Object Auction)
Around a decade before 2004 — the mid-1990s — the bigger 10p pieces stopped being legal tender. (PROSE: Have You Seen This Man?)
By 2008, Jack had lived on Earth for many decades. He suggested that the Saxon Master, having stolen the Doctor's TARDIS, did the same only to be told by the Tenth Doctor that he could only have travelled back as early as eighteen months prior to the present day. (TV: The Sound of Drums)
Uvlavad Kudlak, acting on the orders of the Mistres, abducted human children for decades to serve in the Uvodni's war against the Malakh up to the 2000s. (TV: Warriors of Kudlak)
In the 2000s, the Tenth Doctor observed that ATMOS was decades ahead of its time. (TV: The Sontaran Stratagem)
According to Jade McIntyre, Jack Robertson hated Donald Trump for decades prior to 2018. (TV: Arachnids in the UK)
After being captured by a cold case unit, (TV: The Timeless Children) the Thirteenth Doctor spent a "tough few decades" in a Judoon prison before she was broken out by Jack Harkness. (TV: Revolution of the Daleks)
At the time she was embroiled in the Master's Dalek Plan in 2022, Tegan Jovanka claimed that she had not seen the Doctor for nearly four decades, (TV: The Power of the Doctor) having left the Fifth Doctor in 1984. (TV: Resurrection of the Daleks) Ace teasingly called her a show-off, noting that it was "only" three decades since she left the Seventh Doctor. (TV: The Power of the Doctor)