I don't leave any BTC on exchanges because that would be weird and pointless since I don't trade, but I do hold some Ethereum on an exchange in order to get interest on my holdings.  As far as a ratio if I were to put a number on it...  I'd say the ratio of crypto assets on exchanges as opposed to being held in wallets as opposed to exchanges is just under 1 to 100.  This should change once the mtgox coins are distributed, but I imagine I will either cash out those coins or remove them from the exchange as well.