They say about 15 stories and above makes death extremely likely (like over 95 percent or so, don't quote me on that). In terms of how quick and painless, it all depends on how you land. If you land on yr head, that should be quick. If you land in another position, you might feel pain before dying but in theory, that shouldn't last long. the big issue of course is being Able to overcome yr SI. If you can do that, the fall will be terrifying but it would last that long. just make sure there is nothing to break yr fall on the way down (and of course there is no person you have the chance on landing on).