I can only speak for myself and my goals.
I am a talking-type, so my talking level is usual above the reading, grammar - even listening etc.. and I express myself, even with wrong grammar if I have to. So that might not be what you want:
My minimum threshold always was that I can have deep conversations with a single speaker. (not with a group: That is a whole new level). For example in Chinese, the weakest language I moved on from, I could spent the whole day with Chinese people, and while I wouldn't understand everything and would be lost here and there, I would be able to ask about it in Chinese, and get explanations I could understand.
But I never felt ready, I feel it always was too early, and I often feel that the previous languages are not firm enough..
One thing I noticed is that it is a bad idea to start another language just after you "broke through" that talking barrier for the first time. Keep going for a bit, because if you think: "Oh, I can speak it - great, time for a new language" - that might be wrong. While you studied every day, your level is actually inflated a bit, and once you stop studying it will drop down to its real-, "non-study"-level.
So what happened to me is that I lost my abilities to speak Chinese pretty quickly, within 3 months. I'm reviving it here and there, and I just pop in and out between the "talking comfortably" and "can't talk" zones, and that's where the language has been stuck for a while, because I am still mainly studying Korean.. and it is a real dilemma, because if I focus on Chinese, of course the Korean drops, and since the level there is weaker I feel like I forget things waaay more quicker. So it feels a bit like juggling those too with emphasize on Korean.
Why do I still start a new language..? - it is a mix of two things.
1) - because it is hard: Frustration in the intermediate area maybe, bad experiences (failed tests, ex-girlfriends, weird friends that you misunderstood), lack of motivation or feeling tired, or having to face a difficult thing to proceed (reading or writing Chinese characters, or having to study classical Chinese)...
2) - because you are just immensely curious, or just go with the flow. Here is a Korean friend, lets go. Thoughts like: "I have wanted to do that for years, but if I don't do it now I will never do it" - and then I just start, even if I am not ready. At the end of the day, the ear takes a long time to get used to a language, so even sub-optimal time spent is time spent, and like this an earlier start might be beneficial.
So, who knows whats best? You can also go 80% new language, 20% old language. Which I find to somehow work just fine. It is enough that the old one is not dropping, and enough for the new one to grow.