how far in advance to buy online acropolis tickets?

Unless the sales system changes next year, you will have to wait until April 1st 2025 to buy tickets for May 2025 (pricing changes to summer rates on April 1st)

Great thanks for letting me know. What about Tickets for the acropolis museum, Ancient agora and the national archaeological museum? Do you know anything about purchasing those online and the timing associated with it?

The Acropolis Museum is managed by a private organization. It also has summer/winter pricing on the same principle as public sites like the Acropolis.

In May you don't have to worry about long queues, except perhaps for the Acropolis for which you now have to buy your ticket for a specific date and time.

I stay in Athens for 4 or 5 days almost every year, in May, June or September, I have never seen huge crowds in May in front of the ticket offices. But there are possible strategies, just in case:

• Avoid visiting the Acropolis before the arrival of cruise passengers (before 10 a.m.) or wait at least until 4 p.m.

• Buy your ticket the day before.

• If you buy a combined ticket you can buy it on site in a ticket office other than that of the Acropolis, (for example Kerameikos) there will be fewer people, or almost no one.

The National Archaeological Museum is generally very quiet too, I went back to the NAM last year in early June around 10am, there was zero wait at the ticket office.

There are plenty of maps of Athens and its sites to visit.
I made this one :

https://www.google.com/maps/d/edit?mid=1OhGNGU1LUEgwFTul8u4YEe_tijF8Hqs&usp=sharing

JoLui, could you expand on "Avoid visiting the Acropolis before the arrival of cruise passengers (before 10 a.m.)"?

There's a lot of advice I've read to go early or go late.

Passengers from cruise ships arriving in Piraeus and tourists in organized groups come especially to visit the Acropolis. This can be several thousand visitors who arrive almost at the same time, roughly from 9:00 a.m.
Many visitors left before 5 p.m.

This is why, to avoid the largest crowds, it is preferable to go to the Acropolis as early as possible (from 8 a.m.) or wait until the end of the afternoon.