As a general rule, the Gallery auto-updates every few hours - though the exact timing can vary a bit, depending upon server load.
It generally captures the 14 or 15 listings that are ending soonest - which keys in with most eBay buyers habit of concentrating on items ending within a few hours.
I have found that I generally list in clusters of 12-15 items, spread over an hour or so, with the clusters separated by several hours (such as doing a 'morning batch' and 'evening batch' - or the jump from an evening batch to the next day), so haven't had any particular problem with the way the Gallery works.
I would like to be able to manually select the Gallery product mix - but if we had that, then I would have to spend time 'repicking' it once or twice a day - so we're probably best as-is.