Tayammum Instead of Ghusl
Question :

After doing tayammum instead of ghusl of janabah, I slept or urinated. Should I perform wudhu for prayers?

Answer :

Tayammum instead of the ghusl of janabah compensates for the wudhu. If a minor occurrence, like sleeping or urinating, took place afterwards, and you can perform wudhu, then you must do so. If it is not possible, then you should perform the tayammum instead of wudhu for your prayers.

The Taharah of Tayammum
Question :

If one performs tayammum instead of ghusl of janabah, how long will its taharah last? Is its affect the same as that for the ghusl, so it lasts until the next time one sustains a major occurrence (hadath akbar) like janabah, menstruation or touching a dead body?

Answer :

The tayammum becomes invalid with the elimination of the justification that caused one to perform tayammum. For example, if the patient who performed the tayammum instead of ghusl due to his illness was cured, then the tayammum is invalid immediately and ghusl will be required. The tayammum also becomes invalid also if the justification continued but the individual sustained another major occurrence (hadath akbar).