IECI reports 8.5 million total votes cast, well over 50% of the registered voters. I remember a lot of questions prior to the elections about predicted turnout, and at what level turnout needed to be to be considered "legitimate." Thankfully, no one took the bait to provide a hard number. But I think almost all commentators will accept 8.5 million as "legitimate."
Actually, I don't think you'll hear much about that question anymore. Instead, you'll hear naysayers micro-analyzing the results in Sunni-dominated areas to claim that the elections are illegitimate on that basis. Can you say, "Moving the Goalposts"?