Praise be to Allah, The Lord of the Worlds.
From an Islamic viewpoint, Zakah becomes due on money that has reached the minimum Zakatable amount (Nisaab) after the elapse of one lunar year.
When one of these two conditions is missing, Zakah isn`t due.
This is attested to by the Hadith in which `Aisha (May Allah be pleased with her) said, " Zakat does not have to be paid on property until a year has elapsed over it."{Ibn Majah}. Thus, if the salary is spent on different aspects of life and no amount is saved then it isn`t liable for Zakah.
But, if a portion of it is saved, the savings reach Nisaab, and a full lunar year elapses over possessing that amount, it becomes liable for Zakah. In addition,the Zakah of this amount must be given at the end of every lunar year so long as it equals Nisaab.
It is imperative to give Zakah of each Zakatable amount immediately after the elapse of a full lunar year. However, to set apart for each Zakatable amount a separate lunar year could involve difficulty; thus, it permissible to give Zakah on all the savings according to the first Zakatable amount as it is allowed to pay Zakah of recently owned amounts in advance. And Allah The Almighty knows best.