How well do you have to know a language before you can move on and start learning another language? I don't have an exact answer for you, but we can talk about the factors that go into making this decision.
It all comes down to your personal goals for language learning. Do you want to learn a lot of languages to a more basic level? Or would you rather focus on a few languages and learn them to a really advanced level?
Either of these options is acceptable as long as you are sure it is what you want to do, and I have tried my hand at both of these in Spanish, French, Esperanto, Greek, and Romanian.
