How long can soot sit on walls and belongings before the damage is permanent?

Days, not weeks. Soot is acidic, and it begins etching metals, yellowing paints and plastics, and setting into porous surfaces within the first 24 to 48 hours, with the damage becoming progressively harder to reverse across the first week. That clock is the honest urgency in fire cleanup, and it is why soot removal is sequenced ahead of cosmetic decisions. What you can do while waiting: nothing wet. Dry soot can be removed by the crew's methods; soot wiped with household cleaners is driven into the finish and converts a cleaning job into a sealing and painting job.