Can bounty period be changed? The important question here is whether SE team adjusts such settings individually per site or only for the whole network. Although I have no official source of this, it seems that things related to bounties are network-wide settings. See, for example, herehere, here and here.
replaced http://meta.stackexchange.com/ with https://meta.stackexchange.com/
replaced http://meta.mathoverflow.net/ with https://meta.mathoverflow.net/
replaced http://meta.math.stackexchange.com/ with https://math.meta.stackexchange.com/
replaced http://meta.gis.stackexchange.com/ with https://gis.meta.stackexchange.com/