They started that after the events in Falcon Rising. Being drop-kicked out of the Occupation Zone cost them a lot of troops and prestige, so swift steps needed to be taken to bulk up. One way to do that was to allow some Freeborns into the Warrior Caste.
Given the Vipers' exacting training, only the very best would make it. Even so I would expect most would be assigned to rear-echelon units to free up Trueborns for front-line combat roles.
How long it would continue depends on how long it is necessary (phasing out Freeborn recruitment once the manpower crisis has passed would certainly be consistent with the Viper outlook) and/or if the Vipers are even around after the Homeworlds stopped communicating with the Invading Clans.
Basically, that tidbit was one of the last things we know about what the Vipers were up to. The focus has not really been on the Homeworlds.