This is a generic point, but you can’t write a pan of a film you’ve only seen 60% of, or even 90% of. But I’ve always maintained it’s permissible to say, “I blew this film off after 20 minutes and here’s what led to me to this.” That said (and this is also an old riff), if a film isn’t working I’m fairly sure of this ten minutes in. After fifteen minutes I know it absolutely. After 20 minutes I’m leaning forward and cupping my face in my hands. After a half hour my knees are bouncing up and down. After 40 minutes I’m looking at my watch and calculating how much longer I can stand. And anybody who says they don’t experience similar things while watching an incontestably bad film is being dishonest.