Queen's Gambit Declined: Cambridge Springs Defense, Bogoljubov Variation
It's what Magnus played when he drew his first game with Kasparov back in 2004.
By the way, Magnus's middle name is Øen. I wonder how to pronounce it because in Dutch the word means retard 
easily the longest name for an opening is the Réti Opening: Dutch, Lisitsyn, Rydel-Bloodgood, Polish Gambit, Dalesio Gambit after three moves
Queen's Gambit Declined: Albin Countergambit, Fianchetto Variation (1. d4 d5 2. c4 e5 3. dxe5 d4 4. Nf3 Nc6 5. g3)
King's Knight Variation, Four Knights Game: Spanish Variation, Rubinstein Countergambit, Henneberger Variation (1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. Bb5 Nd4 5. O-O)
King's Knight Variation, Giuoco Piano Game: Giuoco Pianissimo, Italian Four Knights, Canal Variation (1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.d3 Nf6 5.Nc3 d6 6.Bg5)
Queen's Gambit Declined: Tartakower-Makagonov-Bondarevsky System (1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Be7 5. e3 O-O 6. Nf3 h6 7. Bh4 b6)
Sicilian Defence: Open, Najdorf Variation, Poisoned Pawn Variation (1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Bg5 e6 7. f4 Qb6)
King's Gambit Declined: Panteldakis Countergambit, Greco Variation (1. e4 e5 2. f4 f5 3. exf5 Qh4+)
King's Knight Variation, Giuoco Piano Game: Evans Gambit Declined, Fontaine Countergambit (1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 b5)
King's Knight Variation, Giuoco Piano Game: Evans Gambit Accepted, Stone–Ware Defence (1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Bxb4 5. c3 Be7)
Sicilian Defence: Open, Scheveningen Variation, Fischer-Sozin Attack, Leonhardt Variation, Fischer Variation (1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.Bc4 Nc6 7.Bb3 Be7 8.Be3 O-O 9.f4)
Sicilian Defence: Open, Classical Variation, Sozin Attack, Magnus Smith Trap (1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 Nc6 6.Bc4 g6 7.Nxc6 bxc6 8.e5)
King's Knight Variation, Four Knights Game: Double Spanish Variation, Symmetrical Variation, Metger Unpin (1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Bb5 Bb4 5.O-O O-O 6.d3 d6 7.Bg5 Bxc3 8.bxc3 Qe7)
Queen's Gambit Declined: Queen's Knight Variation, Modern Variation, Knight Defense, Cambridge Springs Defense, Bogoljubov Variation, Argentine Variation (1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Nbd7 5. e3 c6 6. Nf3 Qa5 7. Nd2 Bb4 8. Qc2 O-O 9. Be2)
Indian Game: King's Indian Defence, Grünfeld Defense, Schleter Variation, Makogonov Variation, Burille Variation, Opočenský Variation (1. d4 Nf6 2. c4 g6 3. Nc3 d5 4. e3 Bg7 5. Nf3 O-O 6. Bd2)
Ruy López Opening: Marshall Attack, Modern, Main Line, 12...Bd6 13.Re1 Qh4 14.g3 Qh3 15.Be3 Bg4 16.Qd3 Rae8 17.Nd2 Re6 18.a4
Very long
This is the longest
But should we count the numbered moves since they aren't technically a name
We shouldn't I think. Stick with words.
Modern Defense: Standard, Pterodactyl, Eastern, Rhamphorhynchus, Pteranodon, Sicilian Variation.
Sicilian Defense: Hyperaccelerated Dragon, Fianchetto, Pterodactyl, Exchange Variation.
Englund Gambit Declined: Reversed Alekhine, Reversed Krebs, Reversed Mokele Mbembe Variation.
Englund Gambit Declined: Reversed Alekhine, Reversed Krebs, Reversed Brooklyn Variation.
basically 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Na5 6.Bb5+ c6 7.dxc6 bxc6 8.Qf3 Rb8
basically
Uh guys try and stick with words not numbers here it gets confusing
Queen's Pawn Opening: Blackmar-Diemer, von Popiel Gambit?
Nimzowitsch Defense, Scandinavian Variation, Bogoljubow Variation, Heinola-Deppe Gambit
Giuoco Piano Game: Giuoco Pianissimo, Italian Four Knights is pretty long and I play it daily
Modern Defense: Standard, Pterodactyl, Eastern, Rhamphorhynchus, Pteranodon Variation 1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Bc4 Bc5 5.Ng5 Nh6 6.Nxf7 Nxf7 7.Bxf7+ Kxf7 8.Qh5+ g6 9.Qxc5
,
4.O-O Bg7 5.Re1 Nf6 6.c3 O-O 7.d4
Tennison Gambit, Intercontinental Ballistic Missile Variation.Aka ICBM
Queens-Pawn-Opening-Blackmar-Diemer-Vienna-Gunderam-Attack-Kampars-Gambit
The Hyperaccelerated Right Backwards Wayward Pawn Attack Transposed, Defensive Fianchetto Variation