How long it is will depend on how you play. There are about 450 quests in the Legendary Edition (and only about 370 in the standard edition). If you just blow through the main quest, the game won't last very long (maybe 15 hours). But if you spend a lot of time exploring, the game could easily soak up 300+ hours, if not more.
It is one of my favorites. I give it a superb 9.5/10.
I started getting bored of it after my third playthrough. By that time, I had spent a solid 500+ hours in all.
There is quite a bit of looting to be done. Seriously, there is a lot. Especially if you join the Thieves Guild.
The inventory system is very well done. It is easy to manage and track. It is a breeze.
Other points worth knowing: It takes time to get used to. It is a Role-Playing game, so don't expect the game to just hand everything to you. You have to work for what you get.