How long should I mature my mincemeat before making Mince Pies?
Mincemeat originally had meat in it along with suet and a small amount of fruits and spices.  

As the cost of the fruits and spices dropped the quantities started to shift until now, you never see any meat at all besides the suet in traditional mincemeat.
Victorian times they would make the mincemeat in the late fall and set it aside in the root cellar for about 8 weeks but evidently could be held for upwards to 6 months.  The cool cellar and the high percent of spirits didn't cause much harm to them back then.  The longer the better as the fruit and suet would absorb more flavours.

Here's a link to a Google book that has a recipe from 1786: 

http://books.google.co.uk/books?id=zZIEAAAAYAAJ&amp;dq=mincemeat&amp;pg=PA423#v=onepage&amp;q=mincemeat&amp;f=false

It doesn't sound that great but then the really old dishes never were.