Hi Silver,
I believe (could be wrong) I read in a blog some time back that the official length for a GV custom greeting was 30 seconds. However, like you, I too have always run into the 20 second brick wall when creating custom greetings. I've tried it multiple times, but it always limits me to 20 seconds.
This appears to me to be a limit imposed by Google and not by anyone else.
Hope it helps,
pc44