Limit on the gift amount a man can give his wife when he is alive ?See detail below

Shariat decides the amount each heir will receive after death of a Muslim. If a Muslim fears that the amount inherited by his wife (1/4 th according to Shariah), will be insufficient for her sustenance, than during his lifetime (i.e. when he is alive) can he give his wife a certain amount of money as a gift? This gifted money would now belong to his wife on which his wife will pay Zakaat. Is there a limit on this gift amount? What if the man gives his entire wealth to his wife as a gift?