Go into the data file (folder)
World (folder)
Maps (folder)
Campaign (folder)
Imperial_Campaign (folder)
descr_strat (notepad)

Change either of these lines:

end_date 1530 winter
timescale 2.00

Changing the top will make it end later, changing the bottom (to 1.00) will make the game last twice as long.

That helpful enough?