Average length of poly(A) tail in eukaryotes is
Mature mammalian messenger RNAs carry polyadenine tail of approximately eighty to two hundred nucleotides on average, longer two hundred to two hundred fifty in newly synthesized nuclear RNAs shortened in cytoplasm, while yeast tails average seventy to eighty residues. Initial addition of ten to twelve adenines is slow distributive phase, then nuclear polyadenine binding protein PABPN1 stimulates processive synthesis to full length. Tail length influences binding of one PABPC per twenty-seven adenines, forming compact ribonucleoprotein particle. Progressive deadenylation by CCR4-NOT complex in cytoplasm regulates translation and decay. Oocyte cytoplasmic polyadenylation lengthens tails to activate translation.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Poly(A) tail length 80-200; Lodish 9th ed., Regulation of poly(A) length