Fair share = Uma quantia considerável / O suficiente (no sentido de bastante) / A cota necessária (suficiente) / etc.
I've had my fair share of disappointments this month = I have had enough disappointments this month. Any more than this would be exceeding my fair share of disappointments, or, put differently, the amount of disappointments that I can bear.
He's done his fair share of work = He doesn't have to do any more work. He's done enough, or more than he's been asked to.
Australia has its fair share of sex tourists = Australia has high enough sex tourism rates. It's bad as it is and it doesn't need to get any worse than that.