Halachically – how long is a wig allowed to be? Since the whole point of a wig is it shouldn’t look like hair, but nowadays they look even better than hair.
The main idea is that the wig should be Tznius in nature that is, that the hair should not be long and wavy. Rabbi Mordechai Ashkanzi Z”L put out a letter to the Beis Rivka seminary in Kfar Chabad in 16th of Shevat 5773 in which he expressed the position that the wigs should not go past the shoulder length and that the hair of the wig should not be made to look identical to a woman’s real hair.
It is worthy to mention that a wig may be made from real hair as recorded in the Shaltei Giburim on the Rif to Tractate Shabbos and recorded in Poskim to the Shulchan Aruch Orach Chayim 75. See also Likkutei Sichos vol. 13 p. 189 where the Rebbe explains the importance of wearing a wig is even if it looks nicer than real hair, with that said, it must be Tznius in style as mentioned.