This took longer than I thought it would. Clocked in at about 23 hours by the time I was finished with everything. The pst.org guide says 10-20. Probably could have cut off a number of hours but the guide and trophy comments make it sound like a lot of the collectible and kill counts don't roll over--most do, though. If I'd just rushed through the first 3 playthroughs and then saved the grinding for the last one I would have knocked it down to around 15, I think.
The game isn't bad, which is weird because the story is trash, the textures are around what you'd expect for a PSP game, the target locking doesn't work well against groups of enemies, and the grappling hook is inconsistent. Despite all that, running around slaughtering ninjas like crazy is a lot of fun. There's a couple types of stealth kill; the first is to just sneak up on somebody and attack them from above/behind/the side/whatever, and the second is to do a fancy jump-in from a distance attack with a little cut scene (these require energy, but if you have enough you can target up to 10 enemies at a time and just slaughter them all in an instant.) Despite the iffyness of the grappling hook, it's still fun to fly around the map with. There's also an alchemy element which lets you make bombs and potions; the potions can be used to give you a huge boost of speed and strength, which is fun.
There are maybe 10 maps. Around the same number of enemy types. Not a lot of variation, which is made worse if you're going for the plat inefficiently (like I did.)
The basic premise is that you ally yourself with one of three daimyos and do missions for them. Those missions can involve attacking the other daimyos, attacking random bandits, or attacking some ninja group. As you progress through the missions and demolish the other two daimyos story-related missions will also become available. The best way to play the game is to only play the story missions and the missions that hurt the other daimyos--once the other daimyos have their forces demolished enough you can kill them which triggers the story missions to end the game.
Most of the trophies come from the story progression, the difficulty (4), completing each daimyo (3), and seeing three additional ending sequences based on three choices you make during the game. The other trophies are around performing 50 kills with a secondary character, performing X stealth kills, performing each type of stealth kill, etc.. One of the trophies is for creating an ultimate alchemy item and in the process you can create a potion that will let you curb-stomp pretty much everything even on Deadly difficulty since you'll be hitting too fast and hard for the enemy to ever get a chance to retaliate.
Overall the game was pretty fun. I wish the guide had been better because it did end up being tedious by the end. If I hadn't wasted so much time grinding for trophies in specific playthroughs (that rolled over and didn't need to be grinded) then I would have enjoyed it quite a bit more. Still I don't regret the $6.99 Gamestop purchase at all.