Online phone directories are usually kept up to date. At Anywho.com's White Pages, the directory of phone listings is updated weekly with numbers of people from all across the nation.
Whitepages.com, a big name in the world of online phone directories, writes that their listings are updated on a monthly basis. Billions of publicly-available records are ingested monthly and then linked to other public records to create a graph for contacts, filled with the names, numbers and addresses of individuals and families. There are many other online directories available with differing frequencies of updates, but they are updated either monthly or weekly on average.