It would take thousands or millions of years. Stronger flares might do it in thousands.

I’m not a believer in some kind of cataclysmic flare that would rip our atmosphere off in one day. But if one was to calculate how much energy and mass a flare would need to be to do this in days to years, I would guess that the flare would fry all life on earth and the lithosphere would be sterilized much faster than the eventual removal of the atmosphere.

Kind of like being cooked by radiation before the blast wave arrives…. Like terminator 2. But not really. Flares are made of mass, not photons. So you would be pounded by super-fast, electrically charged nuclei of hydrogen, helium and trace metals. AKA Alpha and Beta radiation