The Tax Adviser has my answer. Bolded.

 Sec. 645 election will remain in force for (1) two years if no estate tax return is required to be filed, or (2) the earlier of the date the trust and estate have distributed all of their assets or the day before the later of (a) two years following the date of the decedent's death or (b) six months after determination of the estate's final estate tax liability, if an estate tax return is required to be filed (Reg. Sec. 1.645-1(f)). During the election period