Looked at Italy and they peaked at 919 deaths reported on March 27th. It's been slowly decreasing since but we're still at 542 reported today 11 or 12 days after... So yeah, 21 to 28 days to reduce casualties by 2/3 seems the right ballpark.
Edit : Spain look a bit different, day with the highest total is maybe not the best cutoff point, but that should be fairly similar. They seem to have peaked 3 to 5 days after Italy. France peaked on the 6th or the 3rd of April, hopefully.