After a brief check, I found this figure; 10 km or 6 miles. That all depends on how steep the land is. If you wanna see for yourself, Google 'Japanese tsunami'. They have some live action videos of the destruction caught on camera.
I do not think it possible to give an answer to that as it is all dependent on the topography of the impact point. You may find some interesting facts on tsunami at, https://www.factretriever.com/tsunami-facts
Thank you for your reply Master C and the link you provided. I just wondered if there were a particular distance away from the ocean that would be "SAFE". If there are mountains you just go up to a high elevation. But assuming the land is flat as a pancake where is SAFE? :) Happy Monday to thee! :)
myonemaster 