The status of a vessel that has not been used for 12 months:

Some Poskim[1] rule that after 12 months, the taste absorbed in the walls of a vessel becomes nullified. Other Poskim[2], however, rule a vessel remains forbidden forever due to its absorbed taste, even after 12 months. Practically, in a case that one is in doubt as to the status of the vessel, there is room to permit designating it for meat or milk so long as 12 months have passed.[3] Nonetheless, it is best to Kosher the utensils beforehand, [especially if it is made of Kasherable material]. If it is made of non-Kasherable material [i.e. porcelain], then it should be immersed three times in the Hagalah waters.[4] [However, if there is no doubt as to the vessel’s previous use, then if it may not be used without Koshering it before hand, and if it is not made of Kasherable material, then it may not be used at all.]

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[1] Chacham Tzevi 75, 76 and 80 [regarding Bedieved even with Charif, based on Michaber Y.D. 135 that 12 months suffice for Yayin Nesech vessel, although, Lechatchilah is Machmir], brought in Pischeiy Teshuvah Yorah Deah 122:3 and Shaareiy Teshuvah Orach Chayim 451:1; See Sefer Hakashrus p. 176

[2] Rashba 575; Chaim Sheol 2:38; Panim Meiros 1:31; Teshuvas Rav Akiva Eiger 43; See Pischeiy Teshuvah ibid; P”M 103

[3] Conclusion of Chaim Sheol 2:38 regarding a porcelain vessel bought from a gentile and one is unsure if it was used for non-Kosher that possibly one may be lenient due to a Sfek Sfeika; Seemingly, all the more so would this allowance apply by meat and milk, as it is not considered Derech Bishul and hence is a mere Rabbinical prohibition.

[4] Conclusion of Pischeiy Teshuvah and Shaareiy Teshuvah ibid