Pompeii in One Day or Two

I would recommend the two day option! Pompeii is huge and with new areas opening it is worth spending more time. I loved Herculaneum and while it is a much smaller site I found it more interesting. Note that there are a lot of other sites, villas etc. around Pompeii, not sure which are open to visitors but they look to be worth a visit if you have time.

We spent one afternoon in Pompeii on our way from Naples to Positano. Two days might be a stretch.

You need several hours to do Pompeii, but can do it in one day.

It all depends on the depth of your interest in the sites. Spouse could spend two full days in Pompeii, a whole day in Herculaneum, and still want more time . For those with less interest, all day at Pompeii, and 3/4 of a day at Herculaneum will do. However, with an early start, both can be done in one day. We've done it. But the transportation was all arranged for us which saved a lot of time.

Few people have the time to visit Pompeii twice. I'd point out that individual villas or sights within both Pompeii and Herculaneum can be closed for conservation, research, or whatever, They don't often put this on the websites, and I doubt there is much change in two or three consecutive days. Since there is a Five-Site ticket available, I think it's a better use of your time to visit one or two of the smaller sites (their shuttle or Circumvesuviana), which are VERY different from the two "biggies". At cocktail parties at home, you'll meet very few people who have been to the other sites!

If you really want to immerse yourself, also consider the Villa of Oplontis and the Archeological Museum in Naples. We spent 12 hours over 2 days in Pompeii and didn't see everything, about 4 hours at Herculaneum, a couple of hours at Oplontis as a side trip from Pompeii, and 3 hours at the Archeological Museum, which only got us through the top floor.