In this post in Slashdot, we have a measure on how fast we need to be in providing web services to our users: 4 seconds

4 seconds are the time we wait in a site before marking it as unreliable … forever!!!!

[…] four seconds loading time is the maximum threshold for websurfers. Akamai and JupiterResearch have conducted a study among 1,000 online shoppers and have found, among other results, that one third of respondents have, at one point, left a shopping website because of the overall ‘poor experience’. 75% of them do not intend to ever come back to this website again

Is this number applicable to other kind of apps / services?