Until the End of the Encounter: The effect ends at the end of the current encounter or after five minutes, whichever comes first. [Rules Compendium 226]
Keep in mind that "current encounter" doesn't specify a combat encounter, so you could probably treat it as a duration of "for this scene" in many cases. But yes, there is a hard time-based cutoff for encounter-based durations: five minutes.
Since a round is "about 6 seconds in the game world" [PHB1 266, RC 189] we can extrapolate that a summoned creature would disappear after fifty rounds of unceasing combat... to which I can only say I wouldn't blame him.
The Player's Handbook 1  says that the "effect ends when you take a rest (short or extended) or after 5 minutes," which is basically the same idea but slightly less explicit and potentially open to more abuse (like forcing that poor monster to stick around even longer in your 50+ round fight).