For how long must the clothing be worn prior to the nine days to remove its freshly laundered status?

It suffices to wear the clothing for even a mere moment, and so is the widespread custom. Although, some are stringent to wear the clothing for longer periods, and the following other times are recorded in Poskim: 30 minutes, one hour, 2 to 3 hours.

Sources: Shach Yoreh Deah 389:4 regarding Aveilus that another person can wear it “Some amount of time and not specifically an hour and so is the custom”; Ikarei Hadaat 36 Samech; Misgeres Hashulchan 389 that so is custom; Daas Kedoshim 389:3 regarding Shabbos; Nitei Gavriel Vol. 2 10:1 [unlike Nitei Gavriel Bein Hametzarim 34:3] Other opinions: Some Poskim rule one is to wear it for 2-3 hours. [Levush 389; Rav Poalim 4:29] Others rule one is to wear it for one hour. [Yosef Ometz p. 330; Ben Ish Chaiy Devarim 6; Kaf Hachaim 551:91; Piskeiy Teshuvos 551:17] Others write it is to be worn for a half hour. [Nitei Gavriel Bein Hametzarim 34:3] See Nitei Gavriel ibid footnote 4