I think this is answered on every page, and I figured Ken would post his quick link to the article.
Here is Ken's typical post which is quite handy
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Due to a bad design decision (IMNSHO) B2D folders containing GRT data DO NOT obey the standard overwrite rules. They will not overwrite/reuse existing bkd files until the disk is full or the Disk Space Reserve has been met
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To summarize, the folders will not be deleted/overwrote until the disk/partition they are being stored on is dang near full. At that time, they will clear off back to the overwrite date you have set.