You don't mention which torch you are using, but in my opinion, any more than a couple of minutes, something, somewhere, is goofed up.
I've got better than 25' of 1/2" copper, plus 25 feet of 3/8" T grade rubber hose, at least 50 feet, pressurized to 7 PSI. My personal torch (a Barracuda), takes less than a minute to burn off the remaining fuel in the line when the valves are closed.
If you have a hot head, I can't imagine that it would take more than a minute to burn off the fuel - remember it is high pressure - and it will burn fast.
It almost, and I repeat almost, sounds like a partially frozen valve - you think you've got it closed, but there is some frozen propane in the valve stem, which is allowing some propane to flow past the valve. It doesn't take much of an opening, the spud hole itself is extremely tiny, something like 0.002", so having that kind of gap in the valve, at full tank pressure will continue to release propane until the gap closes and the valve is fully sealed.
I agree with Dale that as long as it doesn't happen again, you should be ok - once is happenstance, twice is circumstance, three times is a habit. Once it hits 3 times, take the tank back to the dealer and tell him what's happening and that you think the valve may be faulty.
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