I have read a few hard analysis papers over the years and the gist is “it depends”. If things went bad, a worst case Kessler syndrome cascading failure. It could take upwards of a year or even more to actually impact the majority of space assets.p, remember “space is big”. And in addition to the time the events would take to unfold, the time they will continue to affect the availability of safe orbits will depend on the altitude of the orbits and the mean particle density. Less dense objects at lower orbits will be more affected by the thin air drag and decay sooner, gradually clearing the orbits over a period of years. Meanwhile the higher orbits will continue to have collisions and junk orbiting for varying degrees of forever.

Overall the situation is best modelled with a pretty messy set of differential equations combining the differing rates of debris creation and decay by altitude alongside the rate of impact events that cause gradual reduction in the mean particle size and orbit.

As for the risk, it’s sort of a chaotic breakdown threshold, hard to model. And unfortunately also hard to predictively put risk estimates on.