The Internal Revenue Manual, excerpted below, states that the special estate tax lien created by I.R.C. Sec. 6324 has an "absolute" life of 10 years from the date of the decedent's death and it CANNOT be extended, unlike the general tax lien created by I.R.C. Sec. 6321:

5.5.8.1 (06-23-2005)
Section Overview Characteristics of Estate Tax Liens

1. This section covers estate tax liens. Before filing a lien on an estate tax case, give careful thought to the advantages and limitations of each type of estate tax lien.

2. In many cases, the general IRC § 6324(a) lien is the best tool to protect the government's interest. It is automatically created when any resident of the United States dies. No recorded notice is required for it to become effective. It attaches to all of the assets that are part of the decedent's gross estate and are required to be reported on Form 706, U.S. Estate Tax Return, and is security for any estate taxes that may be determined to be due. If a probate asset (assets in the name of the decedent at time of death) is transferred or liquidated without payment of the tax, but for the exceptions detailed at IRM 5.5.8.2(2), the lien continues to attach to the asset. If a non-probate asset (property described under IRC § 2034 to § 2042) is transferred or liquidated without payment of the tax, a liability equal to the value of the asset at the time of the decedent's death becomes due from the transferee. A separate assessment against the transferee is not needed. Assets of the gross estate can be sold or encumbered free of the IRC § 6324(a) lien if the proceeds from the sale or loan are used for the payment of charges against the estate or expenses of its administration that are allowed by any court having jurisdiction.

3. A limitation of the general estate tax lien is that it has an absolute life of 10 years. It cannot be extended. Estate tax attributable to an estate's interest in a closely held business may be paid over a 14-year period if an extension of time to pay under IRC § 6166 is in effect which could potentially leave the Service without lien protection for four years if a notice of lien is not recorded before the 10 years have elapsed.

IRM Sec. 5.5.8.1.1 (06-23-2005) also contains a helpful comparison chart of Federal estate tax liens, which addresses the liens created under I.R.C. Secs. 6321, 6324(a), 6324A, and 6324B and how such liens are created, their respective attributes, and the form title under the lien is recorded. NOTE: This chart specifies that the Section 6321 lien attaches to all of the decedent's right, title, and interest in any probate property undistributed when the lien arises, i.e., upon assessment and notice and demand for payment). These liens can be coterminous yet have different SOL and tolling provisions.

You should also beware of the "like" lien that arises under I.R.C. Sec. 6324(a)(2) when an estate tax is not paid when due and a spouse, transferee, trustee, surviving tenant, or beneficiary has possession of non-probate estate assets. I.R.M. Sec. 5.5.8.2 addresses that like lien:

4. IRC § 6324(a)(2) provides that when the estate tax is not paid when due, any spouse, transferee, trustee, surviving tenant, person in possession of the property by reason of the exercise, nonexercise, or release of a power of appointment, or beneficiary, is liable for the payment of the estate tax to the extent of the value of any non-probate estate assets held by, or passing to such person.

5. There is no need to assess the liability against the person liable under IRC § 6324(a)(2). The collection statute under IRC § 6502 applies. IRC § 6324(a)(2) also makes all of the property of such person subject to a lien just like the estate tax lien if their transfer of estate assets divests the assets of the estate tax lien. This ″like-lien″ continues until the IRC § 6324(a) estate tax lien expires or the estate tax is paid.

http://www.irs.gov/irm/part5/irm_05-005-008.html