Purity: Timing of Prayer with Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 704:
If someone whose duty is to perform tayammum knows that his legitimate excuse will remain valid until the end of the time for prayers, or if he loses hope in his legitimate excuse expiring, he can perform prayers with tayammum at any point during the entire length of time for the prayer. However, if one knows that his legitimate excuse will expire by the end of the time for prayers, he must wait and perform the prayer with ghusl or wuḍūʾ. If a person does not lose hope in his legitimate excuse expiring by the end of the time for prayers, he cannot perform tayammum and perform the prayer until he loses hope, unless he deems it probable that if he does not perform the prayer with tayammum earlier, he will not be able to perform the prayer by the end of its time even with tayammum.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)