Create as many timers as masks you are using at the moment. The app is responsible for remembering how much time of use is left for each one. When the time comes, it will send you a notification to change it.
All timers are synced in the cloud with iCloud.
Also on your Apple Watch
In addition to iPhone, SafeTimer is available for Apple Watch. The app is totally independent, you don’t need to have your phone to use it.
Free and Open Source
SafeTimer has been altruistically developed by Chubby Apps to help overcome the health crisis. The code is posted on Github.