Regeneration Frequency For GE GXSH39E

My GE Model GXSH39E regenerates every 3-4 days. My Hardness setting is set to 21.
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What determines frequency outside of the # of gallons of water used?

Can the frequency be adjusted and how if so? The city I live in prides itself for the quality of water they supply to the citizen (clean and soft). Thank you.

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