How long do papers take to review?
No, seriously, how long? I did a little measurement.
I downloaded the 360 most recent papers from Plos One (as of Friday). They are all annotated with submission and acceptance dates, so it was easy to just compute the differences.
The plot below is a histogram (one bin per day) in grey with a Kernel density estimate as a solid line.
The result is it takes about 3 to 4 months to get a paper accepted, but with substancial variance.
Looking at the figure, I had to ask who the poor people were who published that paper which was longest in revision.
Alternative Sigma Factor Over-Expression Enables Heterologous Expression of a Type II Polyketide Biosynthetic Pathway in Escherichia coli by David Cole Stevens, Kyle R. Conway, Nelson Pearce, Luis Roberto Villegas-Peñaranda, Anthony G. Garza, and Christopher N. Boddy. DOI: 10.1371/journal.pone.0064858
Submitted on 29 March 2011 and accepted on 22 April 2013, this paper was 755 days in revision.
The fastest acceptance was only 19 days. However, this being Plos One, it is possible that the paper had been reviewed for another Plos journal, rejected with positive reviews on significance grounds, and had those reviews transferred to Plos One. After this, acceptance followed without a new round of peer review.
This is a gimmick. There is perhaps a paper to be written where this is extended to see what areas of research/keywords/&c matter to acceptance time. If I had more free time I might write that paper.
The code for the above is available on github.
Update: Followup with all PLoS Journals.