Naples lead time for booking the archeological museum
Tickets are on sale for four months in advance. There is availability now for all time slots.
www.coopculture.it is the direct website to buy them on.
Unless it is a national holiday, I see little reason to book the museum in advance. We just walked in the day we were there.
When we were there last week there was about a 30-45 minute line to buy tickets. I think that everyone with tickets in hand skipped the line and walked in. It was muggy and drizzled a little so if I had to do it over again I'd buy tickets online before showing up. As already mentioned, no reason to buy them well in advance - even do it on the way to the museum if you want.