Normally this will take 1-3 days depending on the size of the roof, weather conditions, roof conditions, type of roof and framework.
A good roofing contractor should be able to provide an estimate based on the following factors; type of material used (wood, tile etc.), materials needed (roof tiles, plywood), labor costs per hour or day as well as any additional services required such as waterproofing the attic floor after installing new roof shingles.