In the shortrun producers
are faced with a problem if they wish to increase their output. They can
increase the number of hours their employees work and buy more raw materials,
in other words labour and land (raw materials) are variable in the shortrun.
The amount of factory space and machinery (capital) are fixed as they can't
simply be added. In the shortrun land and labour are variable and capital is
the longrun all economic resources are variable as land, labour and capital
can all be changed. The only variable that is fixed is the level of
technology, but this can be introduced in the very long run, e.g., new IT or
production techniques are introduced.
There is no standard measure for these time periods as they vary
greatly from industry to industry, e.g., a market trader can increase its
capital (the market stall) a lot quicker than ICI (a new chemical plant).