but how much would be ok to keep at an exchange? if i want to trade should i always deposit and withdraw the coins? this will cost a lot or not? i i wont be able to buy/sell quickly.
It would depend on you on how much coin you can afford to loss. You may base it on how many percent of your total bitcoin would you risk in depositing it to an exchange like 10 or 20 or 50% of your total coin. When you put money into something there is always risk so to reduce, minimize or spread the risk, don't put all you coins in one exchange or as what they say don't put all your eggs in one basket.