The time that soil stays good for will vary depending on how the soil is stored, but in general, you have about 6 months to use up opened potting soil before it starts to go off. If you have put potting soil into your garden and used it for plants, it should last for about 2 years before you need to start replacing it or topping it up with fresh soil. If you don’t replace the soil from time to time, you will soon find that your plants do not grow as well, and begin to slow down and look sickly. This is because they aren’t getting the nutrients that they need to produce new growth.