Depends a lot on how the repair area was prepped and sealed. Done right with the right products, filler can last forever. Done poorly, as in cheap bondo slapped directly over rust or unsealed metal joints and you will see rust bubbling and rust juices pushing through in a year or two.

I've used sheet metal screws to hold my sheet metal in place while I do tack welds, but I always remove them and fill in the holes. If he left the sheet metal screws in place that tells me he didn't weld anything and piled the filler on heavy enough to hide the screw heads. These old cars flex a lot in that rear window/trunk area. I see a lot of mopars with cracking filler where guys tried to blend over the seam between the panel that goes in front of the trunk where it meets the quarter panel.

I think you should sell the car now while it still looks good. If you keep it, it'll look good for a year or two, then you'll start seeing some cracking around your trunk/quarter window area and some bubbling in your quarter panels behind and around the rear tires. 5 years from now you will be seriously unhappy with it.