1000000000 was a number.
During his ordeal in the confession dial, two successive copies of the Twelfth Doctor respectively believed that they had travelled "nearly a billion" and "well over a billion" years into the future. (TV: Heaven Sent [+]Steven Moffat, Doctor Who series 9 (BBC One, 2015).)
By the 1970s, the Nestene Consciousness had been colonising planets for "a thousand million years." (TV: Spearhead from Space [+]Robert Holmes, Doctor Who season 7 (BBC1, 1970).)
At the time of its disappearance, Bandraginus V had a population of "a thousand million." (TV: The Pirate Planet [+]Douglas Adams, Doctor Who season 16 (BBC1, 1978).)
In 2157, the colony on Sifranos had a billion settlers, all of whom were massacred by the Daleks. (PROSE: Lucifer Rising [+]Jim Mortimore and Andy Lane, Virgin New Adventures (Virgin Books, 1993).)
In his fourteenth incarnation, the Doctor told Donna Noble that he was a billion years old. (TV: The Giggle [+]Russell T Davies, Doctor Who 60th Anniversary Specials (BBC One and Disney+, 2023). Timestamp 00:19:00.)
Multiple billions[]
According to legend, the Face of Boe had lived for billions of years. (TV: Gridlock [+]Russell T Davies, Doctor Who series 3 (BBC One, 2007).)