How long can you wait to set a broken nose?

1-2 weeks. If a broken nose has made the nose crooked, then it ought to be set within 1-2 weeks after the injury. If you wait 3 weeks or longer then the bone will be too well healed to simply reset it. Beyond that time a more involved operation with "rebreaking" the nose will be needed.
Broken nose. Setting a broken nose should be done urgently (first few days) if there is not a tremendous amount of swelling. If the initial "setting" is not satisfactory from a functional or cosmetic standpoint, a more formal procedure can be done after the nose has healed a bit.