I answered this some 6,223,678,221 pages ago on this thread. "Success" will be when bitcoin (or crypto) gets used by a large number of people because it is cheaper/faster/safer/easier/whatever than the alternatives. Not because one has a BTC stash to burn, not to support the cause, not to evade taxes or make illegal payments, not because it is hip, not to speculate with, not out of curiosity, etc.
That's a pretty reasonable point of view. It's just that with this approach one could never profit from early investments in startups and unverified ideas.
However, by that measure bitcoin has already achieved success. The bitcoin network currently mediates between 50 and 60 thousand transactions every day, transferring an estimated 70 million USD every day.
As far as bootstrapped nascent currency experiments go, bitcoin should certainly fulfill many criteria for "success" already.