Length of time needed varies but can be sped up by agitation, such as bubbling air through the solution, and/or heating the solution. Any precious metals (any metals less reactive than copper such as gold,palladium and platinum) will cement out of solution onto the surface of the copper metal as a black powder. Remove and observe the copper metal daily or several times per day by gently brushing any black material from the copper back into the solution and reinsert the copper metal into solution for a few minutes and then look again. Any fresh black powder formation means you should leave the copper in the solution longer. Continue until no more black material cements onto the copper metal.
When using sulfamic acid, heat the solution to near boiling. Add the sulfamic acid crystals to the solution in very small quantities, at first. If there is excess nitric acid, there will be a burst of activity and if you add too much at one time, the solution could boil over the container. It's best to do this in a vessel that is half full of solution at most or less. Add until there is no more reaction when you add more. You can cover the vessel with a watch glass. A trick I use to let me know when the sulfamic acid is depleted is when red NO2 gas is formed. As long as there is sulfamic acid neutralizing the excess nitric acid, the space in the vessel between the surface of the solution and the watch glass will remain clear with no color. When the sulfamic acid is depleted, red NO2 will begin forming again and you know it is time to add more. It is finished when (1) there is no reaction when more is added, and (2) red NO2 is not formed after being covered for some time.
