Until you die. At which point your heirs get a "stepped up basis" to the date of your death without paying any capital gains tax, under the theory that they pay estate tax instead. But estate tax starts at $10M or so these days. And don't worry if you have more money than that - you've got enough breathing room to create irrevocable trusts to skip estate tax and distribute income directly to your low tax bracket decedents in perpetuity.

For an example of how much the IRS theoretically cares about not being able to kick income down the road - basically the entirety of retirement savings regulations revolve around the controlled ability to do just that.