How long does it take to sell a watch?
In physics, the residence time is a measure of how much time the matter spends in a system. For example, the residence time of lake water is a calculated quantity expressing the mean time that water (or some dissolved substance) spends in a particular lake.
In the watch market, the residence time is how many days a watch is kept for sale on a given Web site. In other words: «How long does it take to sell a watch?»