The Chabad custom follows the Zman Baal Hatanya/Gr”a for Sof Zman Achilas Chametz, in contrast to that of the Magen Avraham.[1] Nonetheless, the Chabad custom, as was followed by the Chabad Rabbeim, was to stop eating Chametz 45 minutes before the start of the 5th hour.[2] Thus, in conclusion, even according to the Chabad custom to follow the later timeframe [i.e. Zman Baal Hatanya], in actuality one should arrange to finish his breakfast and stop eating Chametz close to, and at times even earlier, than the early time frame [i.e. Zman Magen Avraham] in order to abstain from Chametz 45 minutes before the end time. Nonetheless, if for whatever reason one did not manage to eat, or finish eating, Chametz by the end time of the Baal Hatanya, then one may even initially eat until this point.[3]

[1] Igros Kodesh 10:294; Likkutei Sichos 29:378;

[2] Admur in Siddur regarding Kerias Shema; Letter of Rav Yaakov Landau ,printed in Kovetz Yagdil Torah N.Y. Vol. 52 p. 150, that so he was instructed by the Rebbe Rashab, in name of his father the Rebbe Maharash, that even by Rabbinical matters one is to precede himself 45 minutes from the end time, and thus he was told by the Rebbe Rashab to stop eating Chametz 3/4 ‘s of an hour before the time; Rav Eli Landa Shlita confirmed with me that this is the practical Chabad custom that is to be followed [although he said 40 minutes before the time, not 45]

[3] Avraham Chaim Naa’h, printed in Yagdil Torah 6:2604