You don't have to look to the stars for a sprawling game experience in the triple digits. There's plenty to do in a world like Toril, especially in places like the Sword Coast or Amn.
The granddaddy of all PC RPGs, Baldur's Gate 2 (or, if you must, the "enhanced edition" from Beamdog) is custom-made for playing in long stretches.
Even if you've already played it from beginning to end, there's plenty of reason to jump back into the Bhaalspawn saga and try a different route. Side with or against Bohdi and her vampires, go with an all-evil party by grabbing Korgan, Viconia, and Edwin, or try another class to earn a radically different stronghold.
Another option that involves a significant time investment is the Baldur's Gate 2 romance system, which actually plays out over weeks and months of in-game time as you get to know companions.