"At around 400 kilometers and into the 500-km realm — home to ISS and the SpaceX Starlink satellites among others — atmospheric drag plays a major role. Dead satellites and debris usually slow and burn up in the atmosphere in just a few years. This natural cleansing process accelerates when the sun becomes more active and solar coronal mass ejections strike Earth and cause the atmosphere to swell."
Still, if the events occur not once in a decade but multiple times a year that could mean trouble for space travel:
"Linares sees a potential future where 'humans probably don’t have any incentive to launch satellites, because we’re losing 50% of them' to collisions with debris, he says."
Maybe twenty years, after taking out the Starlink constellation and most other satellites below ~400 KM.
I'm sure that we'd still risk unmanned launches to higher altitudes during this time, but manned launches would very likely be restricted, at least by risk-adverse NASA.
A starlink satellite has a life expectancy of only about 5 years anyway, and that is WITH station keeping ion thrusters. Anything smaller than a starlink satellite will likely have an even shorter lifespan due to having worse mass vs surface area ratio because of the square cube law, and should be gone within a few years. Anything larger can easily be tracked with radars etc and should still be gone relatively quickly.
There would need to be a lot of it to overcome continued launches. Once each piece of debris is coming down to that altitude it doesn't have a lot of time left.
