That depends on whether there has been significant hardware changes since last activation. Sometimes if activations have been frequent this can cause problems also. If internet activation does not work then activate it by phone. The process normally runs smoothly. There doesn't seem to be a set amount. My first Win 7 license failed a second activation as I had to reinstall within a week. After activating by phone it has activated on line a couple of times since then.
I cannot agree. I have activated OEM software many times on line and I had to ring up about a retail Office 2010 install quite recently when on line activation failed.