Again as has always been advised by all the great investors John Bogle also suggest don't panic and keep long term in perspective, in fact buy on days where there is big decline.
On Economy: He said this time it is different. He generally do not think much about the Economy and macro environment but this time he is also worried about the economic environment. He thinks valuations are fine this time as compared to what was experienced in past declines in stock market.
His investment advise : Be cautious! but dont fully get out of equities. Have a self discipline of not selling when markets have declined
Here is the video: