As for the Army of Iran (Professional Army) it would take longer then 5 days to defeat them entirely simply because of the logistics involved. We could resupply them fast enough to win in less then 5 days.
However, securing the victory after defeating the Iranian professional Army would be the big concern.
I wonder if we could approach it differently than we approached Iraq:
Hit 'em hard. Destroy the professional army. (Like in Iraq)
Totally destroy the nuke installation.
Spend a month destroying military installations.
Leave 'em alone.
Tell the Iranian people that we can help them build a free society if they invite us in and if they take a leading role in combatting the troublemakers.
If they don't want us there, so be it. But their power will be neutralized.
Securing the peace will be the Big Job. Just like in Iraq.
I don't stay up nights worrying about the Iranian army.
Their main tactic would be to send in suicide tanks faster than we could re-arm the tank and anti-tank crews to blow them up.
We would not occupy the land at all. We would destroy their military from the air and let the people take over in a mass civil war. Iran has been many names in the last 2,500 years.
An attack by Iran on the US would give it yet another new name.