The skin starts out thick, but then becomes thinner, as water evaporates from the surface. The soap mix can be selected to reduce the rate of evaporation.
Due to gravity, water from the top of a new bubble, will tend to flow down to the bottom of the bubble. As water evaporates, that reservoir at the bottom, will provide replacement water to the skin, prolonging the life of the bubble, until it dries out and bursts.
If you see rainbow colour patterns in the surface of a bubble, then the skin is only a few wavelengths of light thick. The colours come from light interference, due to different path lengths between the inside and outside surfaces of the envelope.