First of all make 110% sure you are using a renowned exchange thats of course legit and wont screw you over. If you are watching the market and know you will be buying or seeling inn any moment, id advise to keep just enough in your exchange account in the case of buying since you should have a clearer idea about how much you wish to buy vs what you might want to sell. Keep moving what you wish to hold long term from exchange to wallet(preferably offline hardware wallets or similar since they are safest) and thats that! Its really up to you, hut i personally would not trust anyone with my crypto investment or money for too long or if we are talking huge amounts. Hope this helps