It seems the minimum is once per minute - however restarting wdwebcamcapture has it saving every minute now.
However another problem has surfaced. The DataStead RTSP filter that I installed from niko’s link is an evaluation version that says it stops working after “a few minutes to a few hours”. I didn’t notice it stopping, but it put an “evaluation version” text across the picture. This text disappeared when I restarted wdwebcamcapture, so I’ve ticked the “restart regularly” box in wdwebcamcapture and I’ll see if that restarts it often enough to keep the annoying text away.