If you're talking about KotOR, if you played the Light Side and turned her away from the Dark Side, you live out many more days after the destruction of the Star Forge days until Bastila (along with many other of Revan's original crew) eventually leaves Revan and presumably goes back to her home planet (I believe. Don't quote me on that.). If you played the Dark Side and kept her alive to be your apprentice, then she is your apprentice until Revan either kills her many years down the road after being undoubtably challenged by her, or she will become the next Sith Lord by killing Revan.