How long does it typically take for Sue Grafton to write a book?

In a 1999 interview with Writers Write, Sue Grafton stated that she writes a book every 15 to 18 months. Previously, she strove to release a book once a year. In 2010, Grafton told Writer's Digest it was taking her two years to write a book.

At the time of her interview with Writer's Digest, Sue Grafton had recently released "U Is for Undertow," the 21st book in her Kinsey Millhone alpha-murder mystery series. As of 2015, Grafton has reached the letter X with the release of "X," the first book in the series to deviate from the pattern for the series' titles.