Everybody else will act normal and all prior evidence of their existence will also persist like normal.
I guess for most people it would be pretty much immediately if their significant other isn’t there in the morning and for some fringe isolated cases it might take forever.
I guess best case for me would be around 4 hours on a work day during lunch break seeing a big crowd of people when it would finally click.
Worst case would be probably 5 days when I’m mostly working from home office.
They go in a quantum state, existing and not-existing at the same time until someone perceives them, at which point they go into one of the two quantum superpositions: being or not being.
Or nothing would happen, idfk.