While the main story has something like 58 missions which account for something like 41% of the game completion. The first time I played through this version I beat the main missions plus some other stuff such as destroying cameras and finding drug dealers plus a couple random characters missions and I was at 55% at 25 hours. So it took me about 25 hours to complete half the game. There are still lots of side missions to do after that to keep the game going for a long long time.
As for if you would want to play through a second time that is hard to say. Personally I really like Chinatown Wars. It is one of my favorite GTA games so I do enjoy the story, the dialog is good, and the missions are varied. I never felt tired of doing the missions as they often kept things mixed up and kept you interested in your goals (something other GTA games I feel lack). Honestly if you want to get 100% you will be putting in a lot of hours. After you finish you may want to play through again but you can also just replay the missions in the game from any of your safehouses.
I've played through the DS version, the iPhone/iPod Touch version, and I'm currently playing the PSP version now. So far I would probably say this version is my favorite because I always have my iPod Touch with me making this the ultimate portable version. Also I found that with the DS the touch screen segments always had me reaching to pull out the stylus which kept getting annoying, with this version it was designed with touch controls for your fingers so you don't have to always reach for the stylus. The PSP version so far is nice but I don't like the mini games without the touch screen controls, something just feels off about that..