The index they track reabalances annually [1].

[1] https://am.coinbase.com/documents/cbi-methodology.pdf 6.10

And, since those rebalances reflect only relative changes in supply, the turnover should be a small fraction of AUM.

With a 0% maker and <=0.25% taker fee on GDAX [0], I'd be surprised if you ever came close to paying 2% of your total holdings in fees over the course of a year.

[0] - https://www.gdax.com/fees/BTC-USD

Taker fee is 0.25%

Ah yeah, edited to fix, thanks.

You also have the job of figuring out how much you lose by not rebalancing and then figuring out the interval that maximizes total profit (oh and don't forget to include taxes in that calculation)