What is the meaning of “Real Time” for the web?

I have spent the first eight years of my software development career developing an embedded software for telecom devices. At that point of my life the term real time indicated for me a system that has to reliably and consistently perform a task in a very small a mount of time. For example switching from a faulty controller of a fiber to the backup should be done in less the 50 milli-seconds.

Later I wrote network monitoring software, and discovered the term “soft real time”. Soft real time is about the human perception of time and its granularity. User interface should respond to input in less than 5 second, if your software takes a second to respond that is good, but if it takes half a second, it is most likely that no one will notice the difference. If a router fails it is important to notify the on duty system administrator in no more than a minute, but since it will take him several seconds to just open is cellphone and look at the message, there is no much point to do extra work just to cut the notification time to one second.

Now people are talking about real time updates on the web. To have a meaningful discussion about it, and the proper ways to implement it, someone has to define what is the expected notification time and the longest time which can pass from the origination of a message until people are notified about it. Without a proper definition no one can objectively asses the pros and cons of methods to implement it.