Onion rings are typically made through coating a ring of an onion in a flour-based dip, and then deep frying it. In addition to the flour-based dip, some also coat the ring with dry bread crumbs, hence adding a further layer of Mezonos product to the ring. In both cases, one recites the blessing of Mezonos upon eating onion rings. One recites an Al Hamichya after eating them if he ate at least a Kezayis of the Mezonos coating within within Achilas Peras [i.e. 4 minutes]. If he is unsure, then he should eat two other foods, one of Al Hamichya and another of Borei Nefashos, and then say both after blessings.
 Sefer Viten Bracha 4:3 [p. 79]
Other opinions: Some write the blessing over onion rings is Haadama. [Luach Rav Prus] Seemingly, he compares this to chicken or fish Shnitzel, and other crumb coated foods, of which the custom is to say the blessing of Shehakol and not Mezonos. However, in truth, one cannot compare the two, as the coating of onion rings is thick and a main part of the food, and is similar to coated peanuts which is Mezonos. [See Piskeiy Teshuvos 208:6] Thus, his ruling regarding breadcrumbs is inaccurate, and its blessing is Mezonos.
 The reason: As all mixtures that contain a Mezonos ingredient for purposes of taste, or satiation, receive the blessing of Mezonos even if they are not the main ingredient. [Admur 208:1; Seder 3:2; Luach 4:2; Michaber 208:2; Brachos 36b] Now, certainly onion rings are coated for purposes of eating and tasting the bread crumbs and not just for the onion.
 Four minutes: Shiurei Torah 3:15 [p. 303]; Aruch Hashulchan 202:8; Kaf Hachaim 210:5; Piskeiy Teshuvos 210:1 that so is the widespread custom
Opinion of 6-7 minutes: The Tzemach Tzedek [Shaar Hamiluim 1:8-10] records 6-7 minutes regarding the Shiur of Achilas Peras. [Ketzos Hashulchan 36 footnote 5; 59 footnote 4; Shiureiy Torah ibid footnote 35; Sefer Haminhaghim [English] p. 93 regarding Tishe Beav] In Shiureiy Torah ibid footnote 35 he concludes that if one ate 17 grams in the first 4 minutes and completed 28 grams in 8 minutes, he may say Birchas Hamazon.
Three minutes-Chabad custom: See Sefer Haminhagim ibid and footnotes 342-344 for a quote of various opinions of Achilas Peras, and for a tradition from the Tzemach Tzedek, in name of Rav Hillel Miparitch, that differs from the responsa of the Tzemach Tzedek which placed Achilas Peras as 6-7 minutes and rather places Achilas Peras as minimum 3 minutes and maximum 7 minutes. The Rebbe concludes there in the footnote that one is required to suspect for the above tradition. So also rules Igros Moshe 4:41 that Achilas Peras is 3 minutes regarding Bracha Achrona. Accordingly, one should try to eat the Kezayis within three minutes. However if one ate it within four minutes, and perhaps even within 6-7 minutes, one can rely on the Tzemach Tzedek’s other rulings. Vetzaruch Iyun, as perhaps Safek Brachos Lihakel. In any event one si to be very careful in the above.
Other opinions: See Shiurei Torah 3:15; Sefer Haminhagim ibid and footnotes 342-344; Piskeiy Teshuvos 210:1
 See Admur in Seder 3:2 and 8:3-4
 Admur Seder 3:3; Ketzos Hashulchan 59 footnote 7