the mother has been sitting on it but it rolled out from under her and now the air sac has ruptured so I’ve brought it inside in an incubator fat side up in the hope it either stabilises or at least has some air in the egg for hatchingHas she been sitting on the egg since it was hatched?
I am not experienced with cockatiels, but that egg does look alive to me. Normal incubation period is 18 days, so there is still plenty of time. If she didn't start sitting immediately, it might take even longer before it is ready to hatch.
I recommend patience. If everything is normal, it could be as long as a week or as little as four days. Did something happen to make you worry?