Brown Rats (Rattus Norvegicus) are more efficient at breeding than rabbits and are capable of birthing a large scale infestation from a single pair given that the conditions suited for rapid growth are good (which are subject to a number of factors). This means that there would be no seasonal intervals and as soon as a female has given birth she can be ready to conceive immediately though those rearing factors regarding conditions will be taken into account by female rats. If conditions are less favourable, then breeding may take place in the summer and autumn months.
Brown Rats are capable of giving birth approximately every 3 - 4 weeks meaning that populations may rapidly grow out of control and may only stop or decrease if the availability of resources is out weighed by the population which depends on the premises and its purpose, though resources may provide sustainability ensuring their harbourage. Other than that, a plan of implemented pest control management between the occupants and a pest controller may need to come into play ultimately altering their reproductive pattern if a permanent resolution isn't possible.