Around here, best time to grab it is when you see it, because in another five minutes it might not be there. I was cruising my favorite spot Monday and saw something to big for the car, but i figured i would drop my wife off at the nearby YMCA a few hours later and bring the truck. It was gone when I came back, but I still found a water heater that was put out after my first circuit that nobody else had hit yet. It ended up paying better for less work, so i was happy!
The other day, i passed through an intersection, and two other scrappers were in trucks at the red light on either side of me. This is in the early evening! If you seriously want to catch the sweet stuff, you should just try and drive through residential areas anytime you go anywhere, and if you see something pick it up at that moment.
Good luck out there!
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