This is a general limitation when using TestFlight. Apple probably wants to prevent that Testflight can be used to distribution Apps to users outside of the AppStore, so they enforce a strict expiry policy. The App runs for 30 days and will then stop working. You can update your beta within this 30 day period, so beta testers can update and then get again 30 days. The 30 days limit is documented by Apple and also the TestFlight App will tell the user about this limit. If you think this is not enough, you should tell your beta testers about it as well.
The alternative would be to go the "old way" by distributing your App via iTunes with AdHoc provisioning profiles. This way you need to register the UDIDs for each device yourself, you are limited to 100 devices in total, but the beta will run until the provisioning profile expires (AFAIK these profiles needs to be renewed every year).