Ableton Forum

For instance, imagine a synth, you play a note, that note gets to the Sustain period of the ADSR, the longer you hold the note the more a given parameter in Ableton changes. The parameter change ends when the key is released.

I want to send the audio signal to a return track based on the length of time a note is held down.

m4l: You could create another track with midi in to envelop shaper with a long attack to the main knob of a multimap device. then map the device parameters to the sends and give them separate ranges.

alternatively, you could just explore the evolving parameters of the synth within the state in which the note is held.
or explore any device that has a retrigger button, and conider the depths to which you can make new things happen.

or on a separate midi track with input from your performance track have a midi note in to note out to convert all incoming only send out a single note. then keyboard map that note to a dummy clip in an audio track where you are sending the signal, but set to gate launch mode with varying envelope automations within the clip assigned to the different send levels...