FLYING THE FLAG AT HALF-STAFF: The pertinent section of the Flag Code says, "by order of the President, the flag shall be flown at half-staff upon the death of principal figures of the United States Government and the Governor of a State, territory, or possesion, as a mark of respect to their memory. In the event of the death of other officials or foreign dignitaries, the flag is to be displayed at half-staff according to Presidential orders, or in accordance with recognized customs or practices not inconsistent with law.
In the event of the death a present or former official of the government of any State, territory, or possession of the United States, the Governor of that state, territory, or possession may proclaim that the National flag shall be flown at half-staff." The code also includes other related details including the specific length of time during which the flag should be displayed at half-staff, in the event of the death of a "principal figure"(e.g., 30 days for the death of a sitting or former President, 10 days for the death of a sitting Vice-President,etc.).
The flags should be kept at half-mast for at least a week. However, it depends with your state's governor and what she decides. They might be up for more than a week until the grieving/tribute period is over.