Oaxaca de Juarez is a compact city with an enormous hinterland, and most of the disappointment you hear about it traces back to one planning error. People book three nights, spend them inside the centro, take a single packaged tour that stops at four places in eight hours, and leave believing they have seen the Central Valleys. What they have mostly seen is car parks.
The city itself can be walked in a day and a half. Its real job is to be the place you sleep, eat and come back to while you work outward across a valley floor that holds a Zapotec capital levelled into a mountaintop, a weekly market that predates the Spanish arrival, a cluster of villages that each make one thing exceptionally well, and the densest concentration of mezcal distilling in Mexico. That is a five-day shape. And the order matters far more than most itineraries admit, because here the calendar decides what is even possible on a given morning.
Five days, and what three days costs you
Five nights is the number. It gives you one full day in the city, one market day, one morning at Monte Alban, one run east along the Tlacolula arm, and one flexible day that can go to the craft villages, the mountains, or straight back to a restaurant you want to repeat.
Three days is survivable but it forces an ugly trade. You will have to choose between the region's big Sunday market and the archaeological site, or between the villages and the city, and whichever you drop is the one people talk about afterwards. If three days is genuinely all you have, take the city on the first day, Monte Alban early on the second, and give the third to whichever valley arm matches the day of the week you happen to be there. Do not try to compress the eastern valley and the southern villages into one hired car. They run in opposite directions from the city and the day becomes four hours of driving with photo stops attached.
Seven days is the other good number, and the extra two are best spent on a night in the Sierra Norte rather than on more of the centro.