I had the opposite idea of davido's: if you loop with a sleep, you could extend the loop condition like this:

while (! -e '/predefined/location/time_is_up') {
    do {'your thing'};
    sleep 900;
}
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or, in appropriate places

exit if -e '/predefined/location/time_is_up';
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This would cause the script to end if you create that file.