A second was a unit of time measurement. Though not the smallest such unit, it was still most useful for measuring only short spans of time. According to the Fourth Doctor, 2,678,400 seconds were in a month. (TV: The Brain of Morbius)
According to the Second Doctor, the normal range of human blinking was about once every ten or fifteen seconds. Observing that Tobias Vaughn blinked far less frequently than that, the Doctor found him to be inhuman. Indeed, Vaughn was actually a cyborg. (TV: The Invasion)
Behind the scenes
Although the Doctor doesn't specify, it seems logical to assume that he is speaking of months as measured on Earth, since he's talking to Sarah Jane Smith, a human. In fact, the Doctor's maths is precisely correct for a 31-day month.