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The general rule of thumb is "after about a month, it is acceptable for either party to open that conversation". You can wait longer, but shorter than that (unless you knew each other REALLY well as friends before dating) can be risky or seen as a red flag.
The actual best time to properly define the relationship comes when you find yourself wanting a definition, needing to know for sure what the other person thinks and feels. That's when you should start talking about it, ideally after a month, provided you can talk/text regularly during that month even if you're too busy for lots of dates. Any time after that is also ok, especially if you don't feel an inherent NEED to define the relationship right away and just see how it goes, but even if you are this type of "windflow" person, always be prepared to think about your relationship and what your answer would be if the question is asked to you.1