Duration of postdoctoral positions in pure math
This varies considerably from country to country.  In the US, the "gold standard" is typically a 3 year position (just a quick look at MathJobs reveals ones at Baylor, Dartmouth, Northeastern, and that was just on the first page of listings). Note that some will say that they are shorter with a possibility of extension to 3 years; the extension is generally just a formality. There are even occasionally 4 year positions.  There are also a mix of shorter positions: one semester or one year appointments at institutes like MSRI or IAS are usually intended to complement specific programs, and generally taken as a supplement to a longer term position.  Sometimes there are shorter grant funded positions, though in most cases I know in the US, the department will be able to step in to extend the appointment to 3 years.  "Postdoc" is also a kind of porous category; sometimes departments will hire shorter term positions when they just need someone to teach a few classes which fit somewhere on the continuum of postdoc to adjunct positions.

In the rest of the world, positions this long are much less common, due to some mix of cultural and economic factors, but certainly not unknown.  Generally positions outside the US are funded by grants, and it is hard to get enough funding to support a postdoc for 3 years (in most countries).