So now 6 May (10 days) or 10 May (14 days)? From this report, it appears 10 May was the end of the helicopter dropping missions. At any rate, this contemporary New Scientist says that the actual release stopped much later, on June 6. Elsewhere, I find an account barely mentioning graphite fire (and a linked diagram in pdf showing a sharp drop on 6 May):

Chapter II The release, dispersion and deposition of radionuclides - Chernobyl: Assessment of Radiological and Health Impact

The release pattern over time is well illustrated in Figure 3 (Bu93). The initial large release was principally due to the mechanical fragmentation of the fuel during the explosion. It contained mainly the more volatile radionuclides such as noble gases, iodines and some caesium. The second large release between day 7 and day 10 was associated with the high temperatures reached in the core melt. The sharp drop in releases after ten days may have been due to a rapid cooling of the fuel as the core debris melted through the lower shield and interacted with other material in the reactor. Although further releases probably occurred after 6 May, these are not thought to have been large.

...Although the releases were considerably reduced on 5 and 6 May (days 9 and 10) after the accident), continuing low-level releases occurred in the following week and for up to 40 days after the accident, particularly on 15 and 16 may, attributable to continuing outbreaks of fires or to hot areas in the reactor. These later releases can be correlated with increased concentrations of radionuclides in air measured at Kiev and Vilnius.

_{*Lunatic*, n.
One whose delusions are out of fashion.}