In the original Shoggy, you either beat the game in 15-20 turns and destroy the enemy clans or else you're toast.
Perhaps a bit of an exaggeration. The time scale is considerably affected by which era one starts the game. Any campaign started in 1530 is much more forgiving to a slow pace. Clan infrastructures are minimal, and there's a lot of ebb and flow before some clans start to dominate. One can afford the time to turtle a bit...but not too long The 15-20 turns might be closer to the truth in any campaign started in 1580, where clan infrastructure is well developed to start, and clans can field advanced troops and have a much larger income at the beginning.

I have Shogun 2, but haven't had a lot of time to play. I have much more free time coming in a couple of months, so I plan on changing that