At the very least long enough to allow people to test the new items in order to give proper feedback on how they are working and any issues that might arise with them.   The current limit even with game on fast time advance is barely enough time to get four workers and enough power, metal,  and space for them to even start working on additional systems or any of the new stuff that was added in.   I get the need for a  demo VS the finished product.  Before you had it setup that there was no save option so people could run as long as they wanted but couldn't save.  This filled the demo role and allowed them to give you feedback on everything.   Like the memory leak issues that would only appear after a set amount of time or base size was reached.   I would assume that you have fixed this issue already.   If I was working on a game I would probably follow Wube's example with Factorio.   They kept adding more stuff in the free demo until they felt that "early access" was the route to go then after that it was free updates to anyone who had the early access version until it went to full release.   All the while they built up the fan base around the game while getting all the feedback they could want.  I don't think there is a time limit on the Factorio demo but you get to a point where you can't advance farther in the tech tree due to it being a demo.   But by the time you hit that road block you've played enough to know if you want to buy the game or not.   I know back in the day FPS games would have almost the entire first level as a demo or at least something equivalent to it.  But then again most companies don't do demos anymore of their games so it's hard to say for sure how to go about it.