This is the formula I use. Tone in a flat bottom tray, use as little of the solution as possible, and keep it moving over the print. Lift the print from time to tim and allow to drain to reduce the possibility of stains on the back.
The image should change color as much as it will in five minutes, but the palladium metal continues to replace silver for the next ten minutes or so. If you tone longer than fifteen minutes some bleaching is likely.