I would estimate the game would take 30 hours to complete. It depends how many side quests you complete. You can easily avoid the main quest for a long time because there are so many things to do.

The game begins in the village of Aleroth (sp?) this contains a starter dungeon and several quests. Once you conquer the starter dungeon your free to explore the game.

Just make sure you grab the music fix and the larger fonts for higher resolutions
Credit goes to Raze Larian for pointing those out to me.

If you explore everywhere and do everything, the game can take 50-100 hours. If you are not a completionist, then 30-40 hours would probably be about average (or less playing on easy).

. Actually, you can leave Aleroth before finishing the catacombs (though you should open them and enter once after helping Lanilor). If you progress too far with the main plot quests, though, the ones in Aleroth will be resolved without your help, so you would miss out on the quest rewards (there is also experience and lots of loot in the catacombs).

Raze has given you an excellent reply regarding how long it can take to play Divine Divinity.

As a newcomer to the game I suspect you will find yourself drawn in and captivated once you leave Aleroth. I say this as the initial catacombs sequence can feel a bit of a grind the first time you play the game through. Once you know what is involved in completing this section it is easier.

Still, there are good reasons for completing the catacombs before leaving Aleroth. You need to gain an item that is required later in the game and completing the catacombs is linked to a central quest. Furthermore, leaving Aleroth before finishing the catacombs is tough - really tough. The experience you gain is needed on the road.

In my opinion, this game is anything but basic, but there is an element of linearity in relation to the main quest. However the side quests, characters you meet and the game's humour more than balance this. Divine Divinity is well written and a highly enjoyable experience.