I'm not an expert (and expect to be corrected on details here), but I think it depends on the orbit altitude - if it's low enough to suffer significant atmosphe

I'm not an expert (and expect to be corrected on details here), but I think it depends on the orbit altitude - if it's low enough to suffer significant atmospheric drag, then it'll de-orbit fairly quickly. But if they're at high altitude (geostationary and geosynchronous orbits spring to mind - around 22000-26000 miles), then that's much less likely, although there's a much bigger area to cover at that altitude. But given the damage that even a tiny particle can cause at those velocities, I think it's a real concern with such mass launches.