Sooner than you think. I've heard that there's a CGI film in production that is bringing back a lot of the 1940's actors into a film-noir type mystery with a few of the now deceased famous actors. If I can find some info on it, I'll post it here.
I guess the biggest hurdle was to get permission from the relative's families to use the likenesses. But at what point do you no longer need permission? 20 years? 50? 100?