What is the max time do you think is acceptable for a web script (PHP for example) to execute before it starts to become an annoyance for the user (on average)? I always thought that if the user has to wait more than 1 second for the page to load (this of course after images and css have been cached..this rule really only applies for subsequent requests) they would start to get annoyed.
42 seconds.
Actually, truth is it more comes down to the age and expectations of the user you are catering to. My parents for example, might be much more tolerant of an extended wait, but are also much more likely to press the button again and again, like they will to a crosswalk or elevator button.
That said the real answer is, how long is too long for the user to go without feedback? Even kicking up a quick hourglass logo, or progress bar, can make all the difference in the world for a user. That said, if the service you are providing should be realtime and act like a desktop app, too long is basically "anything perceptible".
So, cop-out answer... it depends. That said, even a "too long" wait, can be overcome by proper UI design and customer interaction.