Less than an hour...
The alum foil neutralizes the rust immediately... I work on a small section, take a wet rag to it and watch the rust run right off...
As I said I continuously keep the alum foil wet...
Here is the same can approx 18 months later. I did nothing to it, but clean it with the alum foil... My garage is uninsulated and unheated.... Living in the PNW approx 80 miles from the Pacific and approx a 1/4 mile from the Columbia River... I think my region of the Country is considered "damp" a good portion of the year.... But I think the alum foil has kept the rust away, quite well....