  This paper investigates how the duration of various code review periods
changes over a projects' lifetime. We study four open-source software (OSS)
projects: Blender, FreeBSD, LLVM, and Mozilla. We mine and analyze the
characteristics of 283,235 code reviews that cover, on average, seven years'
worth of development. Our main conclusion is that neither the passage of time
or the project's size impact code velocity. We find that (a) the duration of
various code review periods (time-to-first-response, time-to-accept, and
time-to-merge) for FreeBSD, LLVM, and Mozilla either becomes shorter or stays
the same; no directional trend is present for Blender, (b) an increase in the
size of the code bases (annually 3-17%) does not accompany a decrease in code
velocity, and (c) for FreeBSD, LLVM, and Mozilla, the 30-day moving median
stays in a fixed range for time-to-merge. These findings do not change with
variabilities in code churn metrics, such as the number of commits or distinct
authors of code changes.
