| For a standard encounter, I'd say about 30-45 minutes tops. Enough time to get into it and enjoy some of the tactics without getting bogged down.
Much shorter than that and I'd wonder why I didn't handwave the battle and move on. If it's a foregone conclusion that the PCs are to win the combat, why bother spending the time going through the motions?
"You face off a legion of kobolds. After several minutes of brisk fighting, you chew through the front lines and face off against the royal guard defending the kobold general." And then the encounter picks up from there.
I've never planned an encounter to be a guaranteed win for the PCs. I don't see what the point would be. Even a standard, short encounter should be worth running through, and for me and my group, 30-45 minutes seems to be the butter zone. | 
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