This depends on the voice you have configured your bot with.
Our own voices do not have this limit.
If you use native TTS, then we use HTML5 speech, which is dependent on the browser.
Chrome on Windows has a bug that limits the max length of the speech.
Firefox and Safari do not have this limit issue. Also Chrome on Mac does not have this issue.
We will look into adding a workaround for Chrome's bug and try to split the text into multiple speech requests.