From RAW, it's clear that a character can ready a spell that has a casting time of 1 action and release the spell when the trigger occurs or directly after the trigger finishes1: Provided, that the character's concentration is not broken.2
Is there any time limit (rounds, turns, etc.) on how long a character can hold a readied spell, as long as concentration is maintained?
Example 1: How long can I wait just around the corner with a readied lightning bolt having a trigger of "I release the spell when a creature (or anything) comes around the corner"?
Example 2: How long can a back row spellcaster hold a readied dimension door with a trigger of "If an enemy breaks through the front line, then I release the spell"? Which would allow the spellcaster to move and attack with a ranged weapon each turn with an escape plan. ("Normal activity, such as moving and attacking, doesn't interfere with concentration." PBR, p. 80; PHB, p. 203)
Under "Adjudicating Reaction Time" in the Dungeon Master's Guide:
Use this rule of thumb: follow whatever timing is specified in the reaction's description. For example, the opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action. [Emphasis added] (DMG, p. 252)
According to the Player's Basic Rules and Player's Handbook:
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken. [Emphasis added] (PBR, p. 72; PHB, p. 193)