Files get a bit longer.

[manus693]

I've tried every variant, and I almost never get exactly the same length. It's 1-6ms longer than the source file. Often 3ms.
qaac64 --no-delay --rate 48000 --cbr 320 "*.wav"

[nu774]

The output of lossy compression codecs such as AAC typically contains both initial delay at the beginning of the file and padding at the end. Since these cannot be removed for various reasons, encoders like qaac include metadata specifying the length of the delay and padding. Players that support gapless playback are expected to remove these portions during playback (i.e., trim the audio).
However, this metadata is not part of the AAC codec itself, nor is it defined in the AAC specifications—it exists only as a non-standard convention.
Players such as foobar2000 support gapless playback, but ffmpeg does not support gapless playback for AAC (it does not remove the padding at the end of the file).