The effects of Curious Colossus's ability last indefinitely. They don't expire during the cleanup step or when Curious Colossus leaves the battlefield.

Curious Colossus's ability overwrites all previous effects that set an affected creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after the ability resolves will overwrite this effect.

Effects that modify a creature's power and/or toughness, such as the effect of Appeal to Eirdu, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness and effects that switch its power and toughness.

If one of the affected creatures gains an ability after Curious Colossus's ability resolves, it will keep that ability.

The effects of Curious Colossus's ability apply only to creatures the target opponent controls as the ability resolves. Creatures they begin to control later won't be affected.