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.

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