httpRuntime executionTimeout value not applied

Question

Hi,

I'm working on Windows 2008R2 with IIS 7.5. In web.config there is config value:

httpRuntime executionTimeout="90"

Now, web request calls WCF service, which is very time consuming (timeouts for WCF are set to 5minutes). Whilst waiting for WCF to finish, web application throws exception:

Exception Type:        System.Web.HttpException
Exception Message:     Request timed out.
Exception Source:      
Exception Target Site: Target Site unavailable

As far as I make it out, it's exception caused by exceeding 90seconds limit. But... I get that exception after about 150seconds. Is it common that request doesn't break instantaneously?

Thanks, Paweł