When can I actually use Bitcoin as a currency rather than a speculative investment asset? That's when I want to buy Bitcoin.
What is "currency" for you? People refer to "currency" all the time as if it was one thing. Cash is one kind of currency, plastic is another, bank account - yet another, savings - yet another. Converting between those is not instant and not free. Bitcoin works OK as a bank account. That is "currency".
(Now that I have tried putting this into words, I see the whole concept of money / currency is more ambiguous than I originally thought. )