It's lengthy but game play is somehow too monotone to keep your busy through out the campain.
To finish the official campaign you must spend about 50 to 70 hours but your interest will fade out after about 20 hours playing. There are plenty reason for that but one of the most obvious is the quest structure of the game. It's all linear. When you come to new region you continue main quest and receive some optional side quests. If u choose not to do side quests right in this part you can come back later but these quests are hardly related to each other and to the main storyline. The reward for them are also quite predictable: some gold, experience or few magic items. One more thing to say is bosses fight in the game. It's mostly a hit point exchange when you can use lots of portions and the ability to teleport back to town while your opponents normally cant heal themselves. I must say it's not very interesting. But since Titan Quest is mere hack and slash so it can be tolerated. In fact there is no puzzle factor in this game. Besides your character development somehow lacks flexibility. You can't really create your own tactics in each situations like in Diablo. That is because though you gain a lot of skills, most of them are not designed well enough to adept to each type of monster or interactions with other NPCs.
Enough to say about the flaws of this game. For a RPG fan (like me) it's still worth trying and brings about some fun thanks to it's Diablo-like styles.