IANAL, but I don't think there is a specific time to do this. On the other hand, if you have users I would err on the side of caution and at least put something simple up. A good resource for a privacy policy and terms of use is CooleyGo:

Terms of Use: https://www.cooleygo.com/documents/terms-use/

Privacy Policy: https://www.cooleygo.com/documents/privacy-policy/