The number of overs in a cricket match determined the length of the contest. Different matches were comprised of different numbers of overs. In the friendly between Cheldon Bonniface Invitation Eleven and the Doctor's Invitation Eleven in 2010 there were 40 overs. (PROSE: Happy Endings)
Behind the scenes[]
A more precise and useful definition of over is not readily discoverable in Doctor Who fiction, probably because it's such a basic term in British culture.