Without overthinking it: Cyndane's body is that of a youngish woman. Longevity is a side effect of channeling the power. Therefore, Cyndane will be able to live a long life.
I suppose it's possible that "old age" is an effect of the soul, rather than the body, but we've seen nothing that suggests this, so I'm thinking it's not the case.
Also, The Dark One's whole schtick is that he's the Lord of the Grave. Seems like if he can't give his followers immortality, he's pretty lame.
For the sake of the discussion anyhow.
I asked the original question because there seems to be some level of metaphysics involved in the ability to Channel and some of the physical is transcended such as in the case of Aran'gar still channeling saidin despite being in a female body. While I tend to think that Cyndane, without DO imposed immortality, would live for roughly 800 years, despite the fact that as Lanfear she had already lived several hundred years. But I'm not convinced that it's also possible that Cyndane might only live for another 500 years, which is roughly how much time she would have left in her original body.
Obviously there is no definitive answer here, but it's a fun little mental exercise for me 
