What idiot shows up at a wine and cheese party without a fake excuse to be somewhere else by 6? "Thanks for inviting us, we have to get the hospital before the breast implant surgery sale ends, cheers!".
IMO, Cheese and wine implies a couple hours, and definitely no more than 3. Totally different than "Fire up the grill and have some brews," That's several hours, in my opinion.
You know you have overstayed your welcome when the host says I have to get up early for work tomorrow. Typically plan on a 3hr event tops.