Final answer:
The 'Time Lag' metric is most useful for measuring the number of days between the first site visit and conversion, directly addressing the timeframe in question.
Step-by-step explanation:
The metric that would be most useful in measuring the number of days that have passed between the first visit to a site and the eventual conversion is Time Lag. This metric specifically tracks the amount of time it takes for a user to convert after their initial visit. It is expressed in days, which directly answers the question of how many days passed. Metrics such as Path Length can show the number of interactions a user had before conversion, while Conversion Value, Top Conversion Paths, and Assisted/Last Interaction Conversions provide other valuable insights, such as the value of the conversion, the most common paths to conversion, and the impact of various touchpoints on the conversion, respectively.