It can change at any time. I remember a few years ago when the lifetime exclusion was only $1 million.

In that case, if the exclusion is unknowable, then there is no reason to be concerned at all.

"History of Federal Estate Tax Laws
The landmark Taxpayer Relief Act of 1997 called for a gradual increase in the estate exemption from $600,000 in 1997 to $1 million by 2006.5 This set the stage for greater increases in years to come.

Estate taxes from 2010 through 2012 were based on the Tax Relief, Unemployment Insurance Reauthorization and Job Creation Act that was signed into law by President Obama on Dec. 17, 2010, but the law was only good for two years. It was supposed to expire on Dec. 31, 2012, so the federal estate tax exemption and rate would have defaulted to the previous number that was in effect.6

This didn't happen. Congress passed the American Taxpayer Relief Act (ATRA) on Jan. 1, 2013, and President Obama signed it into law on Jan. 2, 2013. ATRA was intended to make permanent changes to the laws governing federal estate taxes, gift taxes, and generation-skipping transfer taxes.7

Fast-forward to President Trump, who signed the Tax Cuts and Jobs Act (TCJA) in December 2017. The exemption had been only $5.49 million in 2017. The TCJA more than doubled that to $11.18 in 2018.8

Tax Exemptions and Rates Over the Years
Here's how the estate tax has been assessed down over the years:9

YEAR ESTATE TAX EXEMPTION TOP ESTATE TAX RATE
1997 $600,000 55%
1998 $625,000 55%
1999 $650,000 55%
2000 $675,000 55%
2001 $675,000 55%
2002 $1,000,000 50%
2003 $1,000,000 49%
2004 $1,500,000 48%
2005 $1,500,000 47%
2006 $2,000,000 46%
2007 $2,000,000 45%
2008 $2,000,000 45%
2009 $3,500,000 45%
2010 $5,000,000 or $0 35% or 0%
2011 $5,000,000 35%
2012 $5,120,000 35%
2013 $5,250,000 40%
2014 $5,340,000 40%
2015 $5,430,000 40%
2016 $5,450,000 40%
2017 $5,490,000 40%
2018 $11,180,000 40%
2019 $11,400,000 40%
2020 $11,580,000 40%
2021 $11,700,000 40%
2022 $12,060,000 40%
The heirs of decedents who died in 2010 had a choice. They could use the $5 million estate exemption at the 35% estate tax rate, or they could elect to use the $0 estate tax exemption at a 0% tax rate, coupling the use of modified carryover basis rules.10

The Exemption Can Decrease After 2025
The TCJA is not permanent. It is slated to expire after 2025, and the exemption amount can revert to its pre-2018 level at that time unless Congress acts to renew the legislation or even some of its provisions."

From: How the Federal Estate Tax Exemption Changed from 1997 to Today

/// double post

Note that lifetime gifting, while you are alive, of $18,000 each per year to as many people as you want will reduce your lifetime exclusion. So, even a billionaire can totally eliminate their Federal estate tax by gifting.