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.
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.
