Sparingly, is the answer. Consider Slackbot, Slack's own in-house bot used for setting up new users, amongst other things. Here's the first thing Slackbot says to a new user:
The humble bot
Immediately, the bot's restrictions are outlined and no attempts to hide the fact that it is not human are made. Natural language generation is at its best when used to transform data-intensive constructs such as JSON objects into easy to comprehend phrases.
The Turing Test is a famous test developed in 1950 by Alan Turing to assess a machine's ability to make itself indistinguishable from a human in a text-only sense. Like Slackbot, you should not strive to make your bot Turing Test complete. Instead, focus on how your bot can be the most useful and use natural language generation to make your bot as easy to use as possible.