Expiration of iOS and OS X Builds
Your iOS and OS X builds expire when the expiration date of your provisioning profile, your developer certificate, or your distribution certificate is reached. The longest possible timespan is 1 year if you create your profile & certificate exactly at the start date of your Apple Developer Program membership. Apple can also revoke certificates at any time at its sole discretion.
You can check the expiration date by opening the Certificates, Identifiers & Profiles page in the Apple Developer Portal, then select Certificates or Profiles.
On iOS, users will no longer be able to run ad-hoc and enterprise (in-house) apps once the profile or certificate is expired. You need to renew the profile and/or certificate, then create a new build with Xcode, and upload it to HockeyApp.
On OS X, users will be able to install and run apps even if the certificate is expired, but not if it was revoked by Apple. However, you will need a new certificate to sign updates and new apps.
Read more here.