All of it depends upon the roof quality, surface area, primer type, and paint type. Removing the rust and preparing the surface could take 1-hour maximum. Applying primer and primer takes 1- 2 hours. Drying time for both the primer and paint takes 2-3 days for one layer and five days for the two layers as you have to wait for completing drying of the first layer to apply the second layer. Hence the whole process may take 8 to 10 days maximum.