Yikes, that's not necessary at all. The solution is much simpler.
A dialogue entry node's subtitle will stay visible for the duration of its Sequence. (More info:
Sequences,
Tutorial series)
If a dialogue entry node's Sequence field is blank, it will use the Dialogue Manager GameObject's Camera & Cutscene Settings > Default Sequence. The initial value of the Dialogue Manager's Default Sequence is:
This will delay for a duration based on the text length and the Dialogue Manager's Subtitle Settings values.
Note: If you have changed the Dialogue Manager's Subtitle Settings > Continue Button mode to a value such as Always that requires the player to click a continue button, then the subtitle still stay visible until the player clicks the continue button, even if the Sequence has finished.