First playthrough took me 12 plus hours just exploring areas and going through the story. Second about 6 hours collecting the remaining clues I missed. Third I just rushed through and killed everyone lol. Sure I didn't see every permutation (it changes according to what clues the characters find during that playthrough) but didn't have the interest to play it through more than 3 times myself (that's still 2 more than usual though)
20+ hours for £30 - not bad (especially given I traded it in for £33 lol)
Some but there are a whole lot of decisions and they actually matter so it can change ridiculously. And of course the best part. All the different deaths that can happen