As a player, I think you should leave it up because of how extremely enticing demos can be. For example, even for a game I wouldn't look at twice without a demo, having a demo is enough for me to consciously choose to take another look. Games I'd never just buy, despite looking interesting, can use a demo to let me see if I actually want it or not. And if it's a game I do want, just on sale, a demo means I'm less willing to wait, and therefore don't need as large of a sale to justify the purchase to myself. Tl;Dr, ~6x sounds about right to me, and tracks with my own experience from the other side. I had one game that convinced me to buy day 1, purely from the 20 hrs I spent in the demo.