The average patch “lives” for 3.7 days, which is about the same as the common mosquito.

For Firefox:

"On average, the lifespan of a pre-rapid release patch is 4.6 days, while the lifespans of post-rapid release patches are 2.8 days and 3.4 days for core and casual developers respectively. Therefore, the shortest-lived Firefox patch is a zero-minute patch submitted before April 2011. The review request on the patch was made and self-approved and the actual time delta was 14 seconds. We also calculated the average lifespan of all the patches in our dataset. The average patch “lives” for 3.7 days, which is about the same as the common mosquito."

PS: the lifespan of a patch ends on landing.

From "The Secret Life of Patches: A Firefox Case Study" (pdf), reported by Philip Chee on dev.planning.