R is an incredibly active software project. Since the first code was checked into Subversion back on 18 September 1997, there have been more than 100,000 updates to the R sources by the 20-some members of the R Core Team. As you can see from the activity graphs from the GitHub mirror of the R sources, the pace of development has been frequent and steady over the lifetime of the project:
It's hard to get a sense of the pace of activity from that chart alone, but Matt Dowle (he of the data.table package) published this mesmerizing visualization of R development to YouTube:
Matt created this visualization using Gource, using the aforementioned R GitHub mirror as a data source. It's cool to see all of the R developers moving around the source code tree as it grows over time. If you'd like to drill down into the individual contributors, Romain Francois has some R code and charts (created in 2009) that do exactly that.