A second or two to get it on and a quick pull to get it off (...resists temptation to indulge in Viz and Carry On style double-entendres).
The Ortieb panniers have quick release clips that are opened by pulling on the connected short handle strap. In theory, the clips should also engage 'automatically' when they're aligned and dropped on the rack bars. Its because of this that I'm not a huge fan of the Ortleib pannier quick releases, as they can appear to be engaged on the rack but still not have closed properly. It's only happened to me once and I pulled in at work to discover that the bag was hanging from a single clip. Since then, I've always double-checked that the clips have engaged fully. Apart from this gripe, the mechanism is very adjustable and should fit any rack worthy of the name.