This tweak was made for older versions where Siri would not auto dismiss itself. 

Dismiss Siri after configured timer is set. Dismiss will start count right after Siri is in'idle' mode (not talking or listening)

Source code exists here:  https://github.com/0xkuj/SiriTTL

Options to configure:

• Toggle Siri TTL on or off on demand
• Configure the duration (in seconds) after which Siri will automatically be dismissed

source code is here: https://github.com/0xkuj/SiriTTL