This oil painting and gouache are now mineral pigments, that is, inorganic pigments with good stability. As long as the ingredients in the pigment remain wet, they will never expire.
I forgot that it is no problem to take out the oil paints that have been left in the corner for more than 20 years, so as long as you pay attention to sealing and keeping humidity, you can always use the old oil paintings. Depending on the thickness of the pigment coverage, the time is different. 1 once a week, and there is a special slow-drying agent.