What European country has the longest national anthem?

The Greek National Anthem, 'Innos pros tin Eleftherian' or 'The Hymn to Liberty', is the longest national anthem in the world with a staggering 158 verses.

https://en.wikipedia.org/wiki/Hymn_to_Liberty

The Greek national anthem has 158 verses, I guess that could be the longest.

Do they sing them all regularly?

The longest national anthem I had to endure was the anthem of Burundi. This national hymn, set terribly for brass band, lasted fully six minutes. And we were standing out there in the snow, forbidden to move. (I was on an honor guard greeting five foreign embassadors into Israel - those of Latvia, France, the US, Burundi and Mauritania.) Next to Burundi's anthem, Latvia's seemed like a short, to-the-point breath of fresh air.

The country of Greece.

Greece may claim to have the longest National Anthem in the world. It has no fewer than one hundred and fifty-eight stanzas of four lines each. The poet of the Greek National Anthem, DIONYSIOS SOLOMÓS (1798-1857), was born on the island of Zante and died in Corfu. Written in 1823, he sang the heroic deeds of the Greek fighters for freedom.

http://www.national-anthems.org/anthems/country/GREECE