There is a 3-way machlokes how often he cut his hair - yearly, monthly or weekly. The Navi states that his hair weighed 200 shekel "b'even hamelech". My simple reading is that this emphasizing how very heavy his hair was. Would this not support the opinion of an annual haircut?
Elozor Preil, Bergenfield, NJ
(1) Your question is asked by the Chazon Ish EH 137:1. He writes that one should not be amazed at the opinions that the 200 shekel grew in the course of 30 or 7 days because the Gemara states in Sotah 10a that the hair of Avshalom was created "Me'en Dugma Shel Ma'alah" - similar to the shape in Heaven. Rashi explains that this means that it was a different sort of hair from that which people down in this world usually possess, i.e. Avshalom's hair did not follow the rules of nature.
(2) There are other sources for this idea. The Navi itself (2 Shmuel 14:25) states that Avshalom was the most beautiful man in the whole of Yisrael and was very greatly praised; his body possessing no blemish at all. The Gemara in Nidah 24b relates that Abba Shaul stood up to his nose in the eye-socket of the skull of Avshalom. Here in our Sugya - Nazir 4b Mefaresh DH H"G and 5a DH Ikka uses the verse (Shoftim 8:21) "Each man according to his physical prowess", to describe Avshalom, and writes that Avshalom was very strong.
(3) In addition, one sees in our Sugya (5a) that the Nazir is only allowed to cut his hair when it becomes heavy for him. The Mefaresh 5a DH Ikka writes that because Avshalom was so strong, his hair did not weigh down on him until after 12 months (in other words - even though his hair was so long, he was so strong that this did not bother him). However we can not necessarily derive the Halachah for ordinary people from Avshalom because even though their hair was not so long, nevertheless it may have weighed down upon them in a shorter period.
KOL TUV
Dovid Bloom