Hi,
Just a follow up question from your helpful advice. How long should you leave potting mix in sunshine to make sure the mould is completely eliminated?

Moldy potting soil? It was kept too moist. Just a little drying out will take care of that. If you are concerned, sprinkle a little cinnamon on the soil and mix it in.

Just a little drying out will take care of that.

Although that is true, I know it doesn't mean much to a newer gardener.
--- What the heck does "a little drying out" mean?

When your potting soil is dry enough so that it doesn't stick to your hands, and doesn't stick together because of wetness... that is dry enough. Sunshine is not needed. Sun rays will hasten the death of the mold, but will also hasten the death of good soil organisms in the soil, too.

Cinnamon is sometimes tried as a surface application for certain unwanted fungi that grow at the soil surface. One could only assume that a "sprinkle" would be like a sprinkle of pepper on your food? But that would do nothing mixed into the soil. If you were going to mix it into the soil, say, a gallon by volume, you would need a quarter teaspoon or more.

Mold is a type of fungus and it reproduces via spores. Drying out the soil may put the spores in dormancy but may not kill them. Substituent watering will re-activate them. I strongly advise against using soil that you know has been infested with mold. If you must use it, it is best to heat it up to a high temperature.

But still, the mold should not reappear unless something is out of balance, again. Mold spores are everywhere, and if the conditions are right, the mold will appear whether it's new or old soil.

Subsequent waterings by itself should not trigger mold growth, unless it is the watering that is the imbalance (too much water).