I just checked and gold toners require little time for permanence. Gold toner coats the silver and this uses up the gold very quickly. Longer times will just make the coating thicker and exhaust the toner sooner. And make the colour change greater.
selenium on the other hand actually reacts with the silver which takes longer. The good news is that even weak solution at 1+20 will make a siginficant increase to expected life but without colour change, just some deepening of the shadows. 5 to 10 mins should be plenty even though it won't be to completion. And its far more economical as the capacity is much greater than gold toner.
So if you must tone for permanence with little if any colour change, then use selenium at 1+20. If you make it stronger you risk increased colour change.
p.s. I've used selenium at varying strengths and found 1+20 to be effective for slight increase in dmax without colour change. I really don't like the aubergine colour it can cause from using stronger or extended times.