Answer:
I found that the general rule of thumb is to maintain preheat for a distance that is at least equal to the thickness of the thickest member being welded, but not less than 3 inches in all directions from the point of welding. The reason for this is to ensure that the entire weld area is at the proper temperature to prevent any cracking or other defects in the weld. The preheat distance may need to be increased for certain materials or applications, but 3 inches is a good starting point.