The series says if a Death dies for you, your life is extended by that of the Death's. I always thought this merely means you're living on time you otherwise wouldn't have. For example Misa should have died at around ~18 or whatever, a Death died to save her so now she gets to live longer by the virtue of not dying at 18. I've seen a lot of people say this means she gets her life (18) + Death's Life (presumably hundreds, if not thousands). This would mean Misa is basically immortal, which seems to be contrary as Remu was very unhappy that Misa's life was down to 1/4th. You'd think 1/4th of the life of a Death would still be significantly greater than the average lifespan of a human being.
Dude please call them Shinigami, it's confusing seeing you say a "Death", even the American Manga kept it Shinigami. >_> Oh and spoilers below for those who are watching the Anime. <_<
Anyway my understanding is that most Shinigami are lazy, and generally don't kill people unless they're about to die themselves. In addition to that Human years are not equal to Shinigami years and vice versa. For a Shinigami when they kill a Human they get the remaining lifespan of that Human, so if they're 50, and they're supposed to die in 5 years, the Shinigami gets 5 years of a Human's life. But 5 years of a Human's life isn't 5 years for a Shinigami, otherwise they could kill 10 people who have 10 years left each and never have to worry about killing again. That theory is backed by the odd time spans Shinigami see over people's heads vs Humans who make the deal for the Shinigami eyes.
In Misa's case, at least from my understanding, is that the Shinigami Jealous who sacrificed himself for her in the first case was already close to dying himself, when he was stalking Misa, he wasn't writing any other names down in his book, and he gave his lifespan for her. Same could be said for Rem to a lesser extent, he killed 3 Humans while he was working under Misa, and she got his lifespan as well. Had he not done that, there was no doubt L would have caught Misa and had her executed earlier, as per her shorter lifespan demanded it. SAme with turning in Light, he would have been executed and the following would have happened sooner:
A major spoiler, is after Light was killed for being Kira she commited suicide a year later, so as you can see, she wasn't immortal all the Shinigami sacrifices extended her life 7 years.