I have several unused Milan metro single ticket after I visited Expo in June.
When will the metro ticket expired? I decided to use in next summer with my family.
This answer is solely based on personal experience. Having lived through the paper-to-magnetic ATM ticket transition I can say that the tickets are valid forever provided that:
Indeed I've used paper tickets for several years after the new magnetic ones were introduced, so long as the stamping machines could be found on both metro and trams.
It is safe to assume that the current system used by ATM (the SBME) is here to stay for a while, and that your tickets should still be valid in a year time (August 2016). Should the reading machine fail to accept them due to a faulty magnetic strip or for some other reason, you can always ask a staff member or at an ATM Point for a replacement, provided that the ticket is unused. Quoting from the ATM FAQ page on the SBME:
What should I do if the stamping machine does not accept my magnetic ticket?
Magnetic tickets which are still valid or which have not been used up (carnet of 10 tickets, weekly 2x6, BI4) and which are still valid, for which the stamping machine display shows “Titolo non corretto” (Incorrect ticket), can be replaced at ATM Points or by Station Operators.