How long do you need to visit Ayutthaya?

The historical park sits on an island about 3km by 3km. The temples worth seeing take four to five hours to visit properly, with time between them to eat something and sit in the shade. If you're coming from Bangkok, you realistically have about eight hours on the ground once you account for the 90-minute train each way. That's enough.

What one day covers

The five main temples: Wat Mahathat (the Buddha head in the tree roots), Wat Phra Si Sanphet (the three royal chedis), Wat Chaiwatthanaram (sunset on the west bank), Wat Yai Chai Mongkhon (the big chedi east of the island), and Wat Lokayasutharam (the 42-metre outdoor reclining Buddha). Lunch at one of the boat noodle stalls in the centre of the island. The whole thing on a bicycle, at a pace that doesn't feel rushed.

What a second day adds

Sunrise. The ruins before the day-trip coaches arrive are quieter and cooler, and the early morning light on the chedis is worth getting up for. A second day also gives you time for a longtail boat loop around the island (200-300 baht for an hour), a slower morning with a proper breakfast, and the kind of wandering you don't do when you're watching the clock for a return train.

Ayutthaya has a handful of decent guesthouses and the town is pleasant at night once the crowds go. It's a legitimate reason to stay over, not just a hedge.

The honest answer

Most people who do Ayutthaya in a day come back saying they had a good day and saw what they came to see. Most people who stay overnight come back saying they wish they'd stayed longer. Both are true.

If this is a one-shot visit and you only have a day, take it. If you have flexibility and you're already in the area, a night adds something.

See Can you do Ayutthaya in a day? for how the single day actually breaks down, and our full Ayutthaya guide for which temples are worth your time.