Well, I think TF has to manually turn this on. I don't think it can be automated. Simply because if it could, he would have done so by now. As much as TF loves this site, I can't see him sitting on a web browser all day watching site traffic to turn on or off the search feature.
Plus, with this being a global community, there is no real slack time. When the US is sleeping, Europe is just waking up, and the Western Pacific Rim is in the evening. So while there may be times when there is less going on, I don't think there's much of a 'slow time'.
I'm sure TF could provide some statistics to show users online at any given hour.
I feel your pain, O&S. . . I too miss the search function. It's one of the more useful features here.