By default, Timer alerts will automatically stop after a few beeps. Optionally, you can set your timer alarms to beep continuously until you manually turn it off by doing the following:
In your individual Timer settings at the bottom of the list there is an option for “Continuous Alert”. Slide the button to be green (On).
This will extend the alert when the Timer runs out until you dismiss it.
This gives you the ability to select Continuous Alert on the specific Timers you want it to run on.