The duration of an ailment can be lengthened by "increased ailment duration" effects. 100% increased bleed duration will make a stack of bleed last 6 seconds, rather than the default 3 seconds. "Reduced ailment duration" effects are also possible.
Changing the duration of an ailment that deals damage affects the total amount of damage it will deal, but does not affect damage per second (DPS). This results in its increasing the total damage the ailment deals over its duration.
For example if a stack of Bleed dealt 120 damage over its default 3 second duration, then a 50% increased duration modifier would result in it dealing 180 damage over 4.5 seconds.
Increased ailment effectiveness effects apply to all stat modifiers an ailment grants.
For example, 50% increased armor shred effectiveness would cause stacks of armor shred to reduce armor by 150, instead of the default 100.