Trillian for Windows supports a few different ways to automatically change your presence based on detecting whether or not you are idle:
Set me away based on my mouse and keyboard usage. If enabled, Trillian will mark you as idle after 5 minutes of inactivity and as away after 20 minutes of inactivity. Inactivity is defined by not touching your keyboard or mouse.
Set me away when my screen saver turns on. If enabled, Trillian will mark you away if your screen saver goes on or if you lock your computer.
Changing automatic presence
Click the Trillian button in the upper left of your contact list and click Settings:
Click the Status section and browse the various options within Automatic Changes: