When it is time to breed your budgerigars, if the hen has been flying in the flight with more than the cock and you wish to know the parentage of the chicks, then it is wise to separate her for about three weeks before you are ready to breed her. This will give time for any sperm she may be carrying from other cock birds to be rendered useless.
After this time place both birds providing they are in breeding condition into the breeding cage you have selected. The hen will most likely show interest in the nest box within hours of being placed in the cage. She will probably go in the nest box within the first two days. Sometimes it takes a little longer depending on the condition she is in.
If after two to three weeks , she has not been in the nest box then I would consider changing her for another hen. Keeping in mind the points raised above.