What is the longest palindromic phrase?

Solution 1:

There is no upper limit to how long a palindrome can be. Poems have been written that are palindromes.

Solution 2:

Wikipedia:

In English, two palindromic novels have been published: Dr Awkward & Olson in Oslo by Lawrence Levine (1986, 31954 words), and Satire: Veritas by David Stephens (1980, 58795 words). In French, Oulipo writer George Perec's "Grand Palindrome" (1969) is 5,556 letters in length. In Hebrew, Ghil'ad Zuckermann wrote a 153-word palindromic story called "Lear's in Israel."

For much shorter palindromes: The first place I look for constrained writing is Mike Keith's website, and it has a few short palindromes and a story about them, though neither is as impressive as some of his other work.