He's killed about ten thousand sisters, if he did it at a rate of 8 sisters per day, that would be:
10000 / 8 = 1250 days
Or about 3 years and 5 months.
Could the experiment have been running for that long?
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In a nutshell, Accelerator went through almost 10K sisters indoors. As a result of this, he was able to efficiently kill many sisters.
Assuming he "works" for 12 hours, killing a sister every 8 minutes on average, he would go through 9800 sisters in 4 months.
It is unknown how long the experiments went on, however, seeing as Nunotaba Shinobu has talked to a clone wearing a winter uniform, and that Accelerator has killed the first clone who wore a summer uniform, one can assume that the experiments went for several months during the autumnal, winter or spring seasons—at least the preparations of the clones—before being stopped. Furthermore, several experiments have been executed within the premises of research facilities until being moved during May of the contemporary year.
In May of the current year, the experiments moved to the outside and empty places. This is because of the requirements of the battlefield, as such experiments 1 to 9802 have been performed inside, while the remaining 10198 experiments would be dealt outside.