broken tibia doctor saying put weight on it after two weeks

as the title says a have a non displaced tibia break , i have been put in a long leg straight cast , after my two week check up doctor said i can put up to 4 stone on it to encourage the bones to knit together
i was a bit shocked /and the doctor never said how often or how long i should do it for i have rung the hospital but he is not back for a week
my question is, is it too soon for this as all the searching i have done on the web suggests it is. also i asked him if it was knitting he said it was so why am i being asked to put 4 stone on it
any helpful advice would be appreciated as i dont wont to risk doing damage till doctor is back in case your wondering this in in uk