In the absence of a statement in the notice of election as to the amount of tax to be paid in installments, the date selected for payment of the first installment, or the number of installments, the election is presumed to be for the maximum amount so payable and for payment thereof in 10 equal installments, the first of which is due on the date which is 5 years after the date prescribed in section 6151(a) for payment of estate tax.
26 C.F.R. §20.6166-1