What you have probably seen is 100mm ducting for 'standard' bathroom fans that are typically rated at 83 m3 per hour which state a maximum length of 3m. I have one rated at 113 m3 per hour that it states can be ducted through 100mm up to 9m. Kitchen extractor fans are much more highly rated than 113 m3 so you shouldn't have any problems, but check what the manufacturer states.
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