What invalidates tayammum is what invalidates wudu'.

The Shaykh : It is a substitute for water purification, so as long as a person is unable to use water due to illness or unavailability, tayammum is sufficient and takes the place of water. Based on this, we say that if a man performs tayammum for prayer, he remains in a state of purity until his wudu' is invalidated, and it is not invalidated by the passing of the time. For example, if a person performs tayammum for the Fajr prayer and his wudu' is not invalidated until the adhan for the Zuhr prayer, what happens? He prays Zuhr without performing tayammum again, yes, without repeating the tayammum, because he had already performed it previously and nothing occurred to invalidate his purity. If he performed tayammum for janabah for the first time, does he have to repeat the tayammum for it every time he wants to pray? For janabah? No, because when he performed tayammum the first time, it became equivalent to ghusl.