We have an Alpine doe that kidded in July of 2009 (FF) that wasn't rebred and is still milking...she didn't take last fall, so we just kept her in the string. Only producing about 1/2 gallon a day now, but since she produces the highest milkfat of our girls, we'll keep her milking until we get her bred this fall. I've heard with the Alpines, this isn't unusual....
You can milk her for as long as she will produce to make it worth you while, but most people when they go to rebred them will dry them off for a couple months before the doe kids. To give her a break.
as far as being healthy, it is fine as long as you are able to keep good condition on her. If you are unable to get enough condition on her then she should be dried off sooner and allowed to gain weight.