Is a 9v battery appropriate for a piezo and 1 LED?
A couple things to note:


Get rid of the PWR LED on the Trinket so it's not always drawing current.
Sleep the MCU whenever possible, only waking on interrupts.
Converting 9V to 5V with an LDO (in this case, the MIC5225) implies a 55% efficiency, max, which isn't great. But at least the LDO has a low ground current.


Also, ask yourself if you really need a micro for this task.

If you want a ballpark for runtime, find the capacity of your battery, estimate your current draw, and see what your upper bound on runtime is.