I never walked out of any of the prequels, probably because i had my children with me. But i saw people doing so. TPM had a varying degree of people walking out long before the end. When watching ROTS i was surprised by how many got up and walked out. The cinema was 3/4 full at the beginning and by the time Windu confronted palpatine, the cinema was pretty empty. I think i would have joined them but my kids had all fell asleep ( and my wife for that matter). Funnily enough though, the film that gets all the hate, AOTC, had the least amount of people leaving. It was mainly viewers who didn't go with their kids that left during the three prequels. I think a lot of parents suffered to sit all the way through just for their kids ;)