If they're free definitely get enough for as long as you're in town and decide when you get there.
Unless you do some research ahead and just want to go to some specific stands, 1 day is almost definitely not going to be enough.
If you're random browsing and quick on your feet 2 days is doable, though even then you'll want to miss out some of the specialist areas.
To see everything, at more than just glancing speed, or if you want a more leisurely pace, allow 3 days.
One word of warning - I recall a distinct lack of places to sit down, both at the show and elsewhere in Munich. Comfortable shoes are essential, and when you do see somewhere to set & rest, grab it, as it might be a while before you get another chance.
Around lunchtime, head towards the big corporate stands as some have free food up for grabs!