Once you reach ~2000 TSCs that you mined yourself. Premade ones don't count. Or if you know at least ~3,000 words total.
The longer you wait, the easier it becomes. And the earlier you make the transition, the more difficult it will be. One of the requirements for going monolingual is learning defining vocabulary, or words that appear frequently inside monolingual definitions but aren't common otherwise. Chances are, if you go monolingual later, you won't have to learn so many of them because you'll have learned them from your immersion.
It's very rare for a word in one language to have an exact, one-to-one correlate in another language. Thus, when you learn a word using a bilingual dictionary, by creating an association between it and a similar word in your native language you gain a slightly incorrect understanding of it. If you don't switch to monolingual dictionaries, you'll have to keep correcting the misunderstandings through immersion, which is a process that takes time. No one guarantees that through immersion alone you can undo all false associations.