While graveyards and the like do appear in the anime and games, death is not really ever explicitly dealt with (creepy pokedex entries aside...) So no one could even begin to guess the average lifespan of a Pokemon, since there is simply not enough information given about it.
However, there is one episode in the anime called Just Waiting on a Friend during which it is stated that a particular Ninetales is over 250 years old. But whether this is just a special case, is unique to Ninetales' species (the episode says that the power in Ninetales' tails allows it to live over 1000 years), or an estimate for all Pokemon is pure guesswork.
Also, it wouldn't really make sense if Ghost Pokemon could die, since ghosts are the spirits of the dead (look at Yasmask's pokedex entries for proof of that).
At the end of the day, Pokemon are meant to be timeless, ageless, creatures. Much like Ash :P And since it is a kid's show, having the Pokemon talk about death would be a bit of a downer.
So there is no real answer to this question, but I hope that gave you some food for thought, anyway.
