"Rogoff says he doesn't want to get rid of cash all at once. First, he would phase out 100s, then 50s, then 20s, leaving smaller bills in circulation for the foreseeable future. "I want to have a less-cash society, not a cashless society," he said."
The ECB is currently phasing out 500s, first by removing those it can get its hands on from circulation, and after a few years by declaring the rest no longer legal tender.
IIRC, the US has never declared any version of its old currency not-legal-tender, and no-one would have any problem spending any US banknote that in any way resembles the current ones in size or shape.
https://www.federalreserve.gov/faqs/how-long-is-the-life-spa...
It depends on the denomination. I think $1s have a lifetimes of ~18 months, but it goes up from there. I would imagine $100s have useful lifetimes of at least a decade if not more.