I think the constelations we see are from aproximately 4 years ago, because that's how long it takes for the light to reach earth. And either way, the constelations don't just dissapear, but rather slowly fade away, until the light is too subtle for the human eye to notice. -S-
First of all light is "not" slow 182,282 miles per second. The distance to the nearest stars is thousands of years. That means the light you see left that star thousands of years ago.Not 10 years ago or one hundred years ago.
In my science book, it says that stars are a celestial body that makes light by nuclear reactions, so I guess one day or another, it burn itself up and dies. Always remember that time is space and space is time. Without space there would be no time.