See Code Section 6501(d), and Form 4810. The estate can request a “prompt assessment,” which means IRS has only 18 months to act. This can be done before the estate is closed; the estate should wait for IRS to respond (sometimes they will do it sooner, at least in normal times). I’m not sure how often this procedure is used, because some may be concerned that it will lead IRS to wonder what they are worried about. Also, 18 months is still a long time to wait, although in many cases distributions can be made except for an amount estimated to cover any proposed taxes.