"They will—as the name implies—last forever."
How long is "forever"
They have a variety of warranties, from 10, to only 30 years. Is 30 years "forever"?
"Built to last literally forever with no rot or decay."
From their warranty;
Weather-related wood decay is warranted from ten to thirty years depending on the grade of redwood. The Young Redwood has a ten-year decay warranty. The Mature Redwood has a twenty-year decay warranty and the Old-Growth Redwood has a thirty-year decay warranty.