It really depends on where one was. However, tl;dr, the Spanish Flu ended once people stopped dying in atypically high numbers.

First up, it is important to remember that influenza was not a novel illness in 1918, and an influenza pandemic was not a new event. During the 1918 outbreak, doctors and newspaper writers drew comparisons to the 1889-1890 pandemic, sometimes referred to as the Russian Flu, which carried off some two million victims due to its high morbidity (it was unusually infectious). However, because influenza was a normal part of life, a typical part of what Alfred Crosby referred to as the "background level" of disease, it was given little special attention.

My knowledge is primarily of the Pacific, and the Spanish Flu spread across the region in three major waves, which roughly parallel the global experience. Around May 1918, many islands experienced the first wave of illness, notable for its high morbidity rate. Still, there were few deaths, and the experience did not appear to be altogether unusual. The second deadlier wave of influenza spread across the Pacific in October and November, and two Union Steamships, the Navua and Talune, were chiefly responsible. The Navua sailed the San Francisco to Auckland route by way of Pape'ete, Tahiti, while the Talune sailed from Auckland on a tour of British claims, primarily Fiji, Tonga, and Samoa.

The ships sailed with a clean bill of health-- influenza was not viewed as anything unordinary, so ports were not required to inspect for it, and ships were not required to report suspected cases of it. It was not considered a notifiable disease, and port inspectors were typically on the lookout for yellow fever, malaria, bubonic plague, cholera, etc. Transnational public health institutions such as the Office International d’Hygiene Publique and West Indian Sanitary Convention were not actively tracking or surveilling potential influenza outbreaks.

The easy answer to your question, when does the Spanish Flu end, is that it only ended when people stopped dying. The danger had passed once the town cemetery could keep up with burying the dead and the government began keeping more detailed records. In Pape'ete the outbreak ran from late November, spreading immediately from the Navua among the local population, perhaps aided by armistice celebrations so that the height of the outbreak ran between 26 Nov. until the second week of January. While hard numbers are difficult to come by as government services basically ground to a halt, roughly 20% of the town's population perished in the outbreak. The situation in Samoa, Tonga, and Fiji, visited by the Talune, roughly mirrors Tahiti's. Over the course of about two months, those archipelagos lost roughly 23%, 10%, and 10% of their respective populations. Calls for help to colonial authorities in New Zealand, under pressure from its own widespread outbreak of Spanish Flu, went unanswered and aid only arrived to help with the aftermath of the pandemic.

Not everywhere in the Pacific was so badly affected by the outbreak. In American Samoa, the US Naval Commander, John Poyer, instituted a unilateral and extreme quarantine of the islands. He essentially cut them off from the outside world after hearing rumors of the outbreak. He did this without orders and went so far as to establish armed shore patrols to ensure no one crossed into the islands. Similarly, the French colonial government in New Caledonia instituted a strict quarantine. Unlike some of the other islands, they were well connected to the colonial network of the western Pacific and beyond. As a result, the settler population in Nouméa could read in the newspapers every day about the march of the Spanish Flu toward the island as it appeared in Capetown and then Syndey. The colonial government, reacting in part from its previous experience with the bubonic plague in 1900 (and subsequent flare-ups), quickly instituted a quarantine that applied to all vessels whether they reported influenza aboard or not. The government also passed laws so that if they announced an outbreak had reached the city of Nouméa, theaters and cinemas would close, and mask-wearing would be required.

The strict quarantine of those two islands prevented a single death during that second wave of influenza. Eventually, quarantines were relaxed, and New Caledonia experienced a comparatively deadlier third wave of the Spanish Flu. That outbreak, which began crisscrossing the globe in 1919, circulated for several years, catching communities who had dodged the earlier waves through luck or careful planning. In July 1921, as crowds gathered in Nouméa to celebrate the July 14th holiday (Bastille Day), influenza slipped into the town, and over the next week, more than 5,000 cases were reported. However, the mortality rate remained very low. Local officials were alarmed by the outbreak and quickly sent telegraphs to Australia seeking information on the outbreak. Health officials in Syndey replied that there was no outbreak and that influenza levels were at their seasonal norm. That makes for an interesting point because officials in Nouméa saw this as part of the Spanish Flu and an ongoing pandemic. At the same time, their counterparts in Australia viewed the pandemic as long since concluded.

In either case, I do not think there is a lot of solace one can take regarding our present circumstances except perhaps the expectation that declining morbidity, or perhaps at least mortality rates, are often central to our articulation that a pandemic has finished.

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There are quite a few good books out there on the Spanish Flu now, but here are some that I'd recommend as either interesting, formative, or popular.

• Howard Phillips and David Killingray, eds., The Spanish Influenza Pandemic of 1918-1919 (Routledge Press, 2003)

• Alfred Crosby, American's Forgotten Pandemic; The Influenza of 1918 (Cambridge University Press, 2nd Ed. 2003)

• Svenn-Erik Mamelund, ‘Geography May Explain Adult Mortality from the 1918-20 Influenza Pandemic’, Epidemics, 3 (2011), 46-60.

• Sandra M. Tomkins, ‘The Influenza Epidemic of 1918-19 in Western Samoa’, The Journal of Pacific History 27:2 (1992), 181-197.

• Geoffry Rice, Black November; The 1918 influenza pandemic in New Zealand (Canterbury University Press, 2005)

• Geoffrey Rice, That Terrible Time: Eye-witness Accounts of the 1918 Influenza Pandemic in New Zealand (Hawthorne Press, 2018).

• Niall P.A.S. Johnson and Juergen Mueller, ‘Updating the Accounts: Global Mortality of the 1918-1920 “Spanish” Influenza Epidemic’, Bulletin of the History of Medicine, 76:1 (2002), 105-115.

• John Ryan McLane, Setting a Barricade against the East Wind: Western Polynesia and the 1918 Influenza Outbreak (PhD thesis, University of Otago, 2012).

• Guy Beiner ed., Pandemic Re-Awakenings: The Forgotten and Unforgotten 'Spanish' Flu of 1918-1919 (Oxford University Press, 2021).