The Shinigami's life went to her since he saved her, so she'd have like 1000+ years or more to live, she sacrificed half her life for the Shinigami that gave her the note book (the one that is with her now) to have the eyes, so even though she gave up half her life she's still going to live a hell of a lot longer than anyone else, probably another 1000+ years depending on the Shinigami that saved her life life span which was definitely over 1000 years. That's why Light refused the deal, half of your life is a lot to give if the average person lives to be like 80 years old and he wants to rule a long time.