I do not keep my coins in one exchange to begin with. I dilute them into multiple exchanges and if possible download the wallet for that coin as well. If there is someone out there who trade daily than they can answer this much better because daily traders are buying and selling everyday and they need to have funds on the exchange constantly however if they are keeping their coins there all the time that means they should be really trusting the exchange really a lot.

I do long term investment and when I buy a coin I usually hold it for a long period of time and if you are holding a coin for a long period of time than you can download the wallet of that coin and keep it on your computer which would create a great security for your coins. It doesn't mean nothing will happen to them but it certainly is better than keeping it on a third party website than your own computer.