there's no timer, it's just a Matter of how much chakra they have. To maintain their existence, shadow clones consume the chakra thay have, once it's used they disperse unless they're dispelled by the caster or recieve damage.
At first he was able to produce twenty five clones and could maintain them for half an hour. But gradually, he increased that number to forty five and could maintain them for two hours. Using shadow clones gave him a much needed boost in his learning ability and chakra control.
Book&Literature · manny0101
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Windfall
Anime & Comics · AlphaCentauri0