Do not expect immediate improvement. In fact, the cat might actually scratch some more at first. This is because the mange mites burrow under the skin, and if the treatment kills them (Ivermectin and other insecticides), the remains of the mite stay under the skin as an irritant until the body absorbs them. Shampoos designed to drive the mites away might not have this problem, but may be less effective.
Look for:
The last thing to recover is the fur. It can take quite a while for the cat's fur to grow back. But if the skin is in good condition, you have won the battle, and time will take care of the fur.
If the cat's condition after one week is worse than when you started, something is wrong. If the cat's condition is basically the same, it's still a problem.
Either you need stronger treatment, more treatment, or a different treatment.
I believe that fighting mange is a care against time. A cure that is so slow that you can't see some progress within one week might be too late to save the animal.