How long does ping pong rubber last?

That is a matter of opinion.  Professional players might change their rubber every week, because they need to have the most spin for best results.  Amateurs who play in tournaments might get new rubber once or twice a year.  Basement players might not care that the rubber isn't as sticky and spinny as it once was, but when it gets slick or dried out you can't get any spin.  Hard bats without sponge, just pimpled rubber, can last for years.