T6 heat treatment, penetration depth?

I have a centrifugal fan. The blades on the fan are made of 6061 aluminum. The aluminum blades were heat treated to be 6061-T6 Al.
How do I determine the penetration depth of the T6 layer? If the thickness of the blades is too great I can imagine the center of the blade would not have T6 properties.
Is this information available in any text or is this a mass / heat transfer problem?
Thank you.
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