For me: 1) when I do my tax filing (once a year) but I guess that depends on the law in your country of residence...
and what do you attach as evidence? deposits, withdraws etc?
It is funny to see this. We must understand what the taxes for using BTC is?
There are two cases:
1 IF it is the payment to Creator of your money THEN you have nothing to pay. Because the BTC was not created by Mr. Rothsield's bank. You already have paed to creator of your BTC.
2 IF it is the payment like for your owner ... THEN yur government is criminal "roof" because you are the free and not slave. In this case you have nothing to pay them
3 ... third not exists, nothing else, all other taxes you already pay in EUR...