Recovery is the rate at which Endurance is replenished over time for players and critters. A higher Recovery rate decreases the time between each tick, while a lowered Recovery has the opposite effect.
Recovery is based on percentages, not fixed amounts of endurance. During each recovery tick, 6.667% (one fifteenth) of a player's current maximum endurance will be recovered. Thus, raising your maximum endurance through accolades and set bonuses will also slightly improve the amount of recovery gained each tick.
Player base endurance recovery takes 1 minute to go from 0 endurance to the 100%.
Recovery formulas can be found in the Endurance article.
All critters in the game (from minions to archvillains and giant monsters) share a common base recovery rate of one 6.667% recovery every four seconds, which is effectively 1.67%/sec. Some critters may have powers that modify the recovery rate from the base values.