Start your new career with period length already set to your desire. Simple edit of an XML (created after the first run of this mod) will allow you set common gameplay settings.
Preset defaults will be to 3 day months (ie 9 day seasons), with ai purchasing all off, and snow, ploughing, lime, fertiliser weeds and stones enabled.
Note: This mod will only affect new savegames, and you do not need to load the mod after the first session of your new career (although you can without problems).