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.

Balduvian Frostwaker’s effect has no duration. The snow land is a creature as long as it’s on the battlefield.

If an affected land later loses the supertype snow, Balduvian Frostwaker’s effect doesn’t end. The land will remain a creature.