Continuity mistake: Before he rapes Melanthe, the suitor Eurymachus spills some wine on his shirt, leaving two stains on it. But by the time he enters the room she is in, the spots have disappeared.
Continuity mistake: In the shot of Hermes flying up to get the plant, Odysseus' sword is over his elbow, but in the next overhead shot his sword is halfway between his elbow and his shoulder.
Continuity mistake: In the scene where Anticlea tries to commit suicide, the sand on the beach is white. However, in a wide shot as Penelope yells "No." the sand now appears to be light brown. This shot was obviously filmed at a different time than the rest of the scene, after the tide had gone in and out.
Continuity mistake: After Menelaus reveals to Telemachus that his father is dead, Telemachus puts his hand on the green stripe on the wall, but in the closeup his hand is below this green stripe.
Continuity mistake: When Penelope gives birth to Telemachus, there is a little girl watching with both her hands on her cheeks. Then we see a closeup of her, and only one of her hands is on her cheek. The other is near her chest.
Continuity mistake: When the cyclops is looking at the men's equipment, he sticks a sword into the ground. The angle of the sword changes between shots.
Continuity mistake: Throughout the battle in the throne room at the end, the position of many of the dead suitors changes between shots.
Continuity mistake: During the Trojan war, when Odysseus and Achilles get surrounded by Trojan soldiers, the position of the soldiers change between shots.
Continuity mistake: When Athena says, "Look at them, I fear for you," Polites is laughing at a joke the sailors told him, but in the closeup Polites is eating food out of a bowl.
Continuity mistake: When Penelope is making olive oil she pours a basket of olives into another empty basket. Between shots, the basket with olives moves closer to the empty basket.
Continuity mistake: At the end of the Charybdis scene Odysseus surfaces from the water and grabs onto the mast of the ship with one hand, but in the next shot he's grabbing it with both hands.
Continuity mistake: When the men are leaving the Trojan horse, there is a spear leaning at the center of one of the wheels, but when Odysseus starts fighting, the spear is at the far side of the wheel.
Continuity mistake: The cyclops throws the bowl at Odysseus and it lands on the ground, but it is gone a few shots later as one of the men says "I will kill him now".
Continuity mistake: When the men are about to stab the cyclops in the eye, Odysseus' hands are near the top of the stick. In the next shot they're further down the stick.
Continuity mistake: When Odysseus is telling Calypso about his scar, he rises from the water. In one shot his head is down and his arm is lowered. In the next, his head is up and his arm is raised.
Continuity mistake: When Penelope weaves her tapestry as the suitors wrestle, there is a golden bracelet on her arm, but when she pulls her maid Melanthe away from the window, it is gone.
Continuity mistake: After Penelope says "I am alone now", her head rotates between shots.
Continuity mistake: Odysseus' hair constantly changes throughout the scene where he is talking to Athena on his ship.
Continuity mistake: Aeolus changes position on his chair between shots of him saying "Everyone knows Odysseus, the great hero who built the Trojan horse".
Continuity mistake: At the end of the duel of Antinous against Telemachus, Antinous picks up a stool to throw at Odysseus and the white fur lands almost exactly next to the wine bag. Then in the shot of Antinous laughing, the fur is much further away from the bag.
Answer: The Trojan War itself was a decade long, followed by Odysseus' ten-year journey home to Ithaca, so all told he was gone roughly twenty years.