What Percent is a Market Crash And Signals Selling Index Funds?

Hello. I have a good amount of money in Index Funds. It occurred to me, I don't really know when to pull it out in case the market starts to tank. I am not asking for predicting when to pull it out, but is there a percentage drop in a single day I should definitely consider taking money out? Thinking back to the covid bottom out. All advice welcome! Thank you!