It would take decades.
Richter ’20 [Wolf, "US Dollar as “Global Reserve Currency” amid Fed’s QE and US Government Deficits: Dollar Hegemony in Decline," https://wolfstreet.com/2020/12/31/us-dollar-as-global-reserve-currency-amid-feds-qe-and-us-government-deficits-dollar-hegemony-in-slow-decline/]
The Eurozone and Japan – with the #2 and #3 reserve currencies – normally have large trade surpluses with the rest of the world. This shows that the economy of a major reserve currency doesn’t need to have a trade deficit. The US didn’t have persistent trade deficits until the early 1990s. There were years before then when it even had a trade surplus. The persistent trade deficits didn’t take off until the mid-1990s and exploded from there on amid the rampant “globalization” of Corporate America. The dollar being by far the largest reserve currency has enabled the US to easily finance its big trade deficits, which enables the US to even have those big trade deficits.
If the dollar’s status as top-dog global reserve currency deteriorates a whole lot, it would shake up that equation. At the pace of decline of the past six years, it would take a decade for the dollar’s share to drop to 50%, with other currencies picking up the slack. But for the equation to be shaken up, the dollar’s share would likely have to drop well below 50%.
“Easy does it” is still the motto with reserve currencies. And it works until suddenly, for whatever reason, things go off the rails. But until something does go off the rails, the movements are slow and steady and span decades.