Activating the ability that turns it into a creature while it's already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

When activated multiple times, it will have the triggered ability multiple times. When the animated Plains deals damage, the ability will trigger once for each copy of the ability. If, for example, you activate the ability three times, the Plains will deal two damage, and you'll gain six life.