Youre very correct because no exchange is totally safe from hackers irrespective of how popular or trusted the platform maybe. For me I think exchanges should be used for trading and not as a place to store funds for a long time. Even if you trade consistently, keep only amount you need for trading in your exchange and move your reserved funds and profits into a personal wallet where you can control your private keys.
When you fund your account, risk starts to exist and risk does not appear only after you store your fund on exchange a long time.
In addition, what is a long time?
Is it one week, two weeks, one month, 6 months or one year?
It does not make any sense to say a long time, immediately after you send fund to an exchange account, you begin to have risk of losing your money because of the exchange (they can freeze your account, seize your fund) or by any secure accident like hack.