I'm not sure if there are hard and fast rules for length, however, using ribonucleic-proteins (RNPs) for incredibly short time durations have been shown to be highly accurate, as the editing material has a lot less time to interact with the DNA, precluding anything but perfect matches.
At the end of the day, they will always have failed transformations, but you only need a few to work to start your colony. I'm not sure what the transformation efficiency of top the of line CRISPR is this day, but I know it's growing by the month.
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