Days before first purchase
The 'Days before first purchase' dimension reports the number of days that pass between the first time a visitor reaches your site and when they make a purchase. For example, if a visitor makes a purchase one day after first visiting, then any subsequent visit or event belongs to the "1 day" dimension item.
After a visitor makes their first purchase, they belong to the same dimension item for the remainder of the visitor's cookie lifetime.
Populate this dimension with data
Adobe automatically populates this dimension based on the purchase event in your implementation. If you implement the purchase event on your site, this dimension always works.
Dimension items include the number of days between a visitor's first visit to your site and their first purchase. Each number of days is a separate dimension item, with "Same day" occurring where a visitor's first visit and their first purchase happened on the same day.