It depends a bit on what you mean with "long": The length of the full text, the length of text of a single verse (perhaps including the chorus) or the duration of the music.

If you talk about the full text version of the original poem, the Greek national anthem is certainly the longest: It has 158 verses. However, only the first two ones were officially adopted as the national anthem.

Talking about a single stanza, it depends again on the structure of the text: For example some African national anthems (for example Burundi) have relatively long text versions which are not clearly structured into verses and chorus. Mayby you take a look in the different Lyrics and Sheet music chapters and count the letters  

In many sources, the national anthem of Argentina is mentioned as the one with the longest duration of music. However, the national anthems of El Salvador, Uruguay, Marshall Islands, South Africa and Vatican City State are not much shorter; it might also depend a bit on how fast they are played.

The longest recordings for an anthem I ever heard was a recording of the Mexican anthem (roughly 6 minutes) and of the French anthem (about 10 minutes).