"For as long as" implies duration. For example: "For as long as I have known him, he has been a decent person". "So/as long as" is more like a condition. For example: "As long as you're happy, then so am I". In this example, you could substitute the single word "if" for "as long as". (This is ignoring a particular usage of "as long as" to make a simple comparison. For example: "The building is as long as two football fields end-to-end".)

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