Every bedding will react differently and it depends in the end on the number of rats in the enclosure, the size of those rats, etc. Change the enclosures when you figure it's the time to do so. You'll know best really on that.

I usually like to keep my future breeder females in a group with one or two older, very calm and nice natured retired breeder females. This helps to socialize these females and makes them easier to manage when I'm in there around them and their future babies. Generally the females go in for their first breeding around 4 to 4.5 months of age or 250 grams in weight.

Male rats will pretty much climb on a female from the moment they are able to until about 5 minutes before they keel over dead of old age. Smaller males do tend to sometimes be kicked off if a big mature experienced breeder female rat just can't be bothered to put up with their antics but generally the little males get the job done. Once male rats start breeding they bulk up quite quickly (all those male hormones I would assume).