As someone who knows a bit about environmental remediation, the cleanup will take decades, but it will be safe enough for people to exist there in relatively little time. They'll likely install a vapor barrier, and an asphalt cap to isolate the contamination and prevent interaction with the contaminated soils, then cover that will clean fill and vegetation. There will also be environmental covenants to limit the use of the land until the site is clean. If you want more info about capping, the EPA has a guide.

https://www.epa.gov/sites/default/files/2015-04/documents/a_citizens_guide_to_capping.pdf