If we use the Doma reconstruction as a comparison it might take 2 patches e.g. 5.2 - 5.4 roughly to complete the Ishgard rebuild. Though realistically Ishgard is a fair bit larger than Doma and would take longer to rebuild.

Once auto-demolish hits the housing situation should stabilize or at least it did the previous time when auto-demolish was turned off and then back on again.