(Bronx, New York USA)
My fish has this cotton like growth on his mouth. When I went to the pet store and explained I was sold a packet of tablets that turned the water green.
A day later this cotton is still on the mouth. How long does it take before it disappears?
Your Goldfish has mouth fungus disease.
The treatment you have been sold will be Malachite Green, a synthetic dye used to color paper and fabric, which is why the water has turned green.
It is commonly sold to treat fungal and bacterial diseases.Follow the instructions carefully as it is quite toxic if you over-dose.
I usually use Methylene Blue (another dye) because it is safer.
Your fish should be put in a bare container with fresh clean water as Malachite Green will kill all the beneficial bacterial in the gravel and filter if you are running one.
Add a couple of teaspoons of un-iodized
cooking salt to the water to boost the fish's immune system.
To answer your question, it will take some time to cure the problem.
What will happen is the flesh that the fungus is attached to will repair itself, and the cotton wool like fungus will drop off. If treatment isn't started early enough, mouth fungus can eat away at the lips of the fish resulting in permanent damage.
When it drops off will depend on water temperature and health of the fish generally. I wouldn't expect it to drop off under a week. Fungus is easy to cure, but it just takes a while.
Fungal attacks are brought on by one of two reasons, physical damage to the body surface, or poor water conditions causing stress, which in turn leads to disease.
Feeding your fish some live food will speed up recovery.