Nobody talks about "percentage of tradable life" because this is a pointless metric. You don't see banks publishing prospectus saying "this investment gained x% over y% of its tradable life". %/year is what matters to all investors.

> So if Bitcoin goes down to $30, that's an amazing investment, right? After all, that's a 500% ROI!

Absolutely. Buying at $6 in january 2013, and selling even at $30 would be a 500% ROI and a great investment over 2 years. What is your point?