Until Peppermint 5 is released next may/june(ish) .. can't be more precise than that as there's no fixed release date, iit's released when considered "ready"
Peppermint is released annually, so is based on every "other" Ubuntu/Lubuntu release.
Peppermint 5 will "officially" be supported until Peppermint 6 a year later, but as it'll be based on the Ubuntu/Lubuntu 14.04 Long Term Support release it will continue to receive at least security and bugfixes for the full 5 years.
Think of it like this .. the Peppermint specific repository will stop receiving updates when a new version of Peppermint is released, but as 99% of it comes from the Ubuntu repos, you'll get updates for 99% of it in line with the Ubuntu/Lubuntu release it's based on.
And the 1% from the Peppermint specific repo tends to be mainly just configs, artwork, etc. .. OK, and the Ice application, but nothing that should stop working or is a security risk if it's not updated
As Lubuntu are going fully LTS with 14.04, there's a good chance Peppermint will too .. I don't think it will be too much work (if any) to support the Peppermint specific repo for the full 5 years, not much in there tends to need changes anyway.