Well being the Son of Gaia...Could Thor's strength limit is actually the earth weight itself... Seeing that Thor tried to lift the Midgard serpent whose weight is the earth weight.. Full Powered Thor's strength is earth's weight.
It's impossible to determine the weight of the earth serpent. It can coil around earth -several- times, but this doesn't mean it weights as much.
Thor's strength, like many with strength far over the class 100 limit, is limited at the whims of the writer. He'll be as strong or as weak as the story requires him to be. Which usually goes from slightly stronger than Thing/Namor, to supporting the weight of a score planets and so on.
Don't even think he was wearing his belt of strength at the time, either.
Thor is pretty much as strong as anyone below skyfather or abstract level being could be.
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"I'm not smart so much as I am not dumb." - Harlan Ellison