how many 80lb bags to cover approx 60 sq ft 3.5 in deep.......how long do i have after mixing to pour and work it before it starts to harden

You will need approximately 30 of the 80 lb. bags of High Strength Concrete Mix to fill this area. It should be poured and then screed immediately. The working depends on temperature and humidity. The warmer it is the quicker it will go and the colder it is the slower it will go. You want to wait and see when the material starts to give up its bleed water before you finish the surface and this time will vary. The material takes around 6-8 hours to set.