I've seen people set-up 3 hours ahead to get one of the benches at the train station. By the castle, facing the castle, in front I've seen two hours because we were one of those people. Most who do this send someone out for snacks, drinks or meals and they use the time to eat while waiting. Some use themselves laying their feet or whatever to save extra spots while the kids do more riding on the attractions, etc. It's hard for kids to wait this long.
I'm a picture taker and this time out I'll be doing the video camera bit; so, I know these waits will be necessary for the best spots. If you don't have to be this fussy then the waits in less popular areas are shorter.
I think an 1-1/2 hour would be good for front row on Main Street. Some of the other areas like by Frontierland you could probably do good with an hour wait.
Weather can be a factor, too. If there's a downpour, you may be in luck. Sometimes with cold weather and some not prepared with coats, etc. you can get a good seat without waiting, as long.
You'll get different opinions on this but my approach is more or less sure proof.