Well, the actual act does only take ~2 seconds. But there's elaborate courtship (sniffing, ear wiggling, etc.) and chasing that leads up to the act, so that's probably where they got the 20 mins to 1 hour figure.
The amount of time is also dependent on whether the other rat's scent is familiar - if the rats live in the same room but in separate cages, it'll probably happen more quickly than if they're complete strangers.
They could also mean the entire process. Courtship, mating, and then fertilization of the eggs.
Have you contacted any of the breeders I directed you to, yet? Like I said before, make sure they are good breeders. You probably won't want to work with anyone who doesn't do it for the right reasons, the right way, etc.