I've never known any ATO investigation to go on for decades. Nine months is the norm.
Not even Wickenby went on for decades.
Maybe Thomson/Williamson will set the record. Who knows?
If it's going to go on for "decades" as you suggest then Gillard may have problems rousing a vote at the next election. No-one likes to vote for someone with a dark cloud above them, not even Gillard apparently.