Right time is when you think your project solves the same problems others may be facing and you think you need an extra help in furthering your efforts or when you have lost interest in the project
From a completion standpoint, an MVP sufficient to convince people that it has value
When you think what you've created can lead to bigger & better things
It depends on your project.
For most things, as soon as one feature is functional. In some edge cases (i.e. games), when you think it's ready for primetime :)
The right time to do it? Well, when you don't have any time for the project.