Average time alloted for manuscript review across disciplines
For mathematics, here's the latest survey done by American Mathematical Society:

http://www.ams.org/notices/201410/rnoti-p1268.pdf

You can find median times from submission/acceptance to acceptance/print/electronic print for various math journals as well as other statistics like the current estimate of waiting time for each journal. Not surprisingly, the median time from submission to acceptance in 2013 varies greatly; some take only a few months and others nearly a year and a half.

They do this survey every year. For instance, here's the one published 2 years ago:

http://www.ams.org/notices/201210/rtx121001473p.pdf

The worst offender that took the longest between submission and acceptance that year was Annals of Mathematics (which happens to be among the most prestigious math journals), and the median was 24 months.

I don't know if there is data for other fields. But American Physical Society occasionally makes statistics for their journals available, e.g., pages 14-17 of this PDF slides:

http://www.phys.nthu.edu.tw/~colloquium/2009F/T2.pdf

From my own experience as an author and reviewer as well as from what I hear, it appears that math journals typically take longer than physics journals. But probably things are drastically different across subfields even within one discipline (e.g., theoretical vs. experimental).

In any case, as the fact that they publish statistics indicates, both disciplines seem to take turnaround time very seriously. But for some reason, it appears that review tends to take more time if a journal publishes more mathematical papers. This seems to hold true for electrical engineering, too; the journal I have published in most frequently belongs to electrical engineering but is known for being heavily mathematical, and, lo and be hold, it's notorious for its slow, slow, slooow review...