LOUISE: How long has it been [that Rory and Dean have been together]?
RORY: … About a month.
LOUISE: Oh, lifers.
It’s quite clearly been more than “about a month” since Rory and Dean got together. It’s been more than two months since they went to the school dance together, so Madeline and Louise must know it’s more than a month.
By Louise describing a couple in a one-month relationship as “lifers”, we can tell she’s never been in a relationship for more than a few weeks at most, and that she views (or affects to view) a long-term relationship in terms of a prison sentence.