Mark Kermode has a policy of not walking out of a film - no matter how irritating, nausiating or downright terrible - unless it features actual animal cruelty. The closest he has come to doing so that he has mentioned in the time of the Film Review Show has been Blindness, which features a particularly harrowing rape scene. This is on the grounds that it would be unfair to judge a movie without watching it all the way through, since you should never review a film you haven't seen. (That said, he does not stay right until the end of the credits, as Philip French of the Observer used to).
Mark's stand-in Andrew Collins did walk out of Crank 2, during the scene in which various strippers' silicone breast enhancements got shot and exploded. (Mark incidentally stayed, and was rewarded with a scene in which Geri Helliwell appears on a quiz show as The Stathe's mum.)
Nonetheless, walking out remains amongst the wider moviegoing public the ultimate act of registering how appalling they think a film is, and many Wittertainees over the years have correspondent to say they have done so. Meanwhile, the question remains of how long one needs to stay with a bad film to make up one's mind. After all, how many films that start out dismally ever really get better? (Especially these days, when things like Leapy Ear show the entire movie in the trailer). There has in fact been discussion - not least on the Kermode Uncut blog - on precisely this issue. Mark has ultimately concluded that, although he would not be changing his policy, the only example he could really think of was Mamma Mia.