[matteopey]

I've been there in July, visited them both in the same day: Herculaneum in the morning and Pompeii the afternoon. Is impossible to visit everything in Pompeii only the afternoon, in fact we went only in the most famous places but we completely missed the most northern part. I think that in a full day is possible to visit a lot of it, but it depends how much you want to stop and listen to the audioguide (highly recommended). Herculaneum is fine visiting it in the morning, you can enter on each and every house, read the textbook that explain a lot about it, etc. A few advice: prepare yourself for a lot of walking under the sun, those places are really hot (especially in July). In Pompeii there aren't many places to rest in the shade since almost every house have no roof, but there are some fountains where you can fill a water bottle.

On another note you should really spend 2-3 days in Naples, a city which I found (unexpectedly) very interesting and fascinating.

Finally you should really make this card [0], it allows you to take every train, metro, bus, funicular railway, from Naples to Sorrento and allows you to enter for free in the first two cultural sites that you visit. So I recommend you to go first in the two Roman cities and save a few money that you can invest in good pizzas :).

[0]: https://www.campaniartecard.it/campania-3-days/?lang=en