Personally, I believe you will have more sales success if you don't put a time limit on the demo. That's because the demo is more than just a "game" demo. It is a marketing tool. You want it on as many PCs as it can get to, and you want lots of people to try it. You want the demo to PERSIST in the market place....ever growing with each download...expanding to new customers...growing....growing.
Given a certain percentage of those touched by the demo will buy the demo, maximizing the numbers touched will maximine the sales percentages accordingly.