I don't think it will ever happen. because currently the total supply of ethereum is more than 114 million with a market cap of 147 billion. and at least it takes 4x more than now to beat bitcoin. and it took years to do. and that's with a note that the price is still the same as it is today.

because that's why the probability is so low. bitcoin will remain the crypto coin with the highest market cap of all time.