Coffee Parts part number is 516528-N, It is listed in group seals as a Brasilia seal, might as well get a replacement shower head (526100) as well, just in case there is a drama getting it out. If you go to domestic machines (not spares) and scroll down, appropriate parts are below. Less than $30 all up including freight.
As to when, if it has gone hard, or is leaking, or you destroy it taking out the shower head for cleaning/servicing, or you want to replace it. But I am sure there are other opinions.