The wearer of a suit of magic armor that possesses the special ability undead controlling (DMG 219) (49,000 gp; 0 lbs.) can take a standard action to employ an effect like the spell control undead [necro] (PH 214) as if cast by a level 13 wizard (hence a Will save DC 20) except that no matter how often this effect is employed over the course of the day the wearer can't control more than a total of 26 Hit Dice of undead creatures and that controlled undead creatures are freed of this control at dawn.
Thus the armor special ability's control undead effect has always had a 13-min. duration, which makes the be-careful!-control-expires-at-dawn! limitation a little weird. (This reader speculates that the special ability's author may have mixed up the duration of the spell control undead with the duration of the 2nd-level Sor/Wiz spell command undead [necro] (PH 211), which is a healthier 1 day/level). Likewise, it's also weird that the special ability has no limit to how often it can be activated, but after a certain point—that is, when the day's Hit Dice cap is reached,—the special ability does nothing if it's activated again.
However, this DM doesn't see any problem with the special ability—even allowing the PC to have a head-up display that counts down his armor's Hit Dice controlled today cap seems a reasonable accommodation given the special ability's price—, and the special ability remains playable enough as written despite its quirks. I mean, I guess eventually even that 26-Hit-Dice lich will roll a 1, and you'll control it for the next 13 min.; use that time wisely… and try to time things so that the lich fails its saving throw more than, like, 1 round before dawn.
Note: The armor special ability undead controlling was absent from the Dungeon Master's Guide (2000), first appearing as a +4 bonus in 2001's Defenders of the Faith (23). The ability was added to the Dungeon Master's Guide by the 3.5 revision, its description largely unchanged from its original appearance.