According to TechNet, when the TimeoutSec parameter of Invoke-WebRequest is ommitted or specified as 0, the timeout is indefinite. There appears to be a bug that causes this value to default to 100 seconds, at least in PowerShell 4. I presume this is because the default value of the underlying System.Net.HttpWebRequest.Timeout property is 100,000 ms.

The workaround is to specify a large value for TimeoutSec, e.g.

Invoke-WebRequest -Uri -TimeoutSec [int]::MaxValue

That is, assuming that you want the request to return within 68 years.