You owe when you use it. While it sits there it's just an investment but if you make any purchase with it you've basically taken out the investment as income. One loophole might be using Bitcoin to buy gold or silver which counts as an investment and in most cases isn't taxable. Since the gold/silver can't be tracked the way Bitcoin is you could spend it and not pay the tax, although I believe you would technically still owe it since you've converted an investment to usable income.