I would say Never even if I'am new on Bitcoin ... but I see that it was about 600$ last month ... and it's dropping badly this month 480$-520$ so .. yep 
You don't have to focus just on the last month or on where we're now...
Take a look to the Bitcoin history, to the charts, to the usability of BTC and you're going to see their potential... trust me
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Let's see what is going to happen but i cna just say one thing: treat Bitcoin and markets with humility