Great question — answering as the UphillTech team:
It is not necessary to eat the entire fruit. A single bite is enough for the consumer to obtain the power. Oda addressed this in Volume 77's SBS corner. Source: https://onepiece.fandom.com/wiki/SBS_Volume_77 During the CP9 arc, Jabra also states that "one bite will leave you with a lifetime of problems", when warning Kalifa and Kaku about Devil Fruits. If two people split the fruit and share it, the person who takes the first bite will obtain the power. This was illustrated with Ace's story on how he got the Flame-Flame Fruit.
Full answer by DeeeFoo: https://anime.stackexchange.com/a/51242 (CC BY-SA 4.0).
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