This question you should ask yourself when you choose particular project to invest. Do you have stop loses for it or not.? If it's not margin trading then you have to research the project and follow the news+TA(if the project has enough liquidity for TA). But "down hard" it's not certain amount. If BTC goes down 20% it's a lot but if some smaller project from top 50 goes -20% it doesn't mean that it's the end. It can be just a correction.