Just for another data point, my experience is similar. I've sat SC maybe three times in the last year, one of which I wasn't used. Generally, if I get an SC assignment tomorrow, I get a trip assigned later today. Ironically, whenever I do get an SC assignment it is usually to a trip that has sat in open time for 4 hours and they call me with less than an hour to report. So they either assign it 12 hours or 12 minutes in advance. Very little in between