Can you base a film review without watching the whole film?
 

This isn't so much of a carry over from the "Walked out of a movie" (because some of them were right) but more along the lines of how much of a film do you need to see before you are confident you can stay/leave/switch channels.

I'm currently 30 minutes into the remake of Get Carter and haven't found it to be as terrible as everyone else has made out.

Of course, I may post back in 5 minutes and say that I was totally wrong lol.

But generally, how much of a film do you have to suffer before it dawns on you that your time would have been better spent watch paint dry?

It depends how I've acquired said film and how much I have invested in it.
If I've bought it on DVD, or paid for a ticket to see it at the cinema, I've invested a few bob and will probably wait it out and hope for the best even if it's not shaping up to be a good film.

If it's something I'm watching on TV or SKY or sent from Loveshite, that puts it into disposable 'take it or leave it' category. I will give it between 20 - 30 mins to interest me. If it hasn't done it in that time; it's out. I don't give one whether a purist would argue that I can't make an informed decision by watching only a part of it. Yes - I can. I've seen so many films that my instinct guides me and I'm fine with that. Life is short, I don't have time for crap, or for something that isn't doing it for me. I know that some of the stuff I reject is not poor quality, but it's not my scene.

Half-hour rule. If a film keeps my attention past the first half an hour then i'll watch it to the end. Dull opening scenes are a death sentence for a film.

YES. pretty much what I said, except with a budget on words.
I agree, dull opening scenes are a killer. That has me reaching for the remote quicker than anything else. Kubrick said something like "The opening frames of a film should grab the audience, immediately" and I've never forgotten that. If you can't be arsed to wow me right off the bat, why should I wait for things to improve?

There are genres I avoid: pretentious indie movies and found footage horror.
Found footage horror is lazy, lame and unscary, sending a message that you don't need film-making craft to do horror, just shaky cam and a few things moving about. Thanks, Wair Bitch Project :mad:

At the end of the film, how can you judge something without viewing the whole movie?

You can't judge an entire movie without seeing the entire movie, sure.
But I can judge, within a relatively short time, whether I consider a movie to be worth my time in watching it, in its entirety. That is what I judge and, since there is a wealth of films out there to watch, I don't lay awake at night worrying if I missed something in a film which I judged to be a piece of crap.

I think I may rephrase the question to say:

"Have you ever switched off a movie and then lay awake at night wondering if it got better JUST after you switched it off?"

Or

"Did you regret your missed opportunity to walk out of a crap film because you thought it would get better?"

I stop a film if the first 5 minutes offends all 6 of my senses (6th one is behind your left ear). Life's too short to waste an hour and a half on a piece of crap.

I don't think there's ever been an instance where I've soldiered on through a film and found that the finale made up for a limp opening. If it starts bad, it's almost certainly going to end bad. I've stopped bothering now, the amount of dross I saw in the two years I owned a Cineworld Unlimited Card has seriously dampened my enthusiasm for seeing a film through if it fails to grab me in its opening act.

I completely agree. I think when you've watched an absolute shitload of films, because you're getting on in years, it becomes easier and easier to tell if a film is going to float your boat fairly early on, or if it's not.