What is the amount of zakat due on money deposited in the post office that has reached its nisab, and is it permissible to use its profits to support the family in the absence of work?

What are called the benefits resulting from a postal savings account are considered forbidden usury (riba). First, it is obligatory to repent to Allah Almighty from that forbidden transaction. And it is obligatory to get rid of those usurious profits by spending them on public interests and on the poor and needy. You are not allowed to benefit from them unless you are poor. As for the zakat on the principal deposited money: It is obligatory to pay zakat on it when a full Hijri year has passed and it reaches the nisab. The amount of zakat is a quarter of a tenth, i.e., 2.5%. Each time a year passes on it, you should give out a quarter of a tenth as zakat. If you have not paid its zakat for the past two years, it does not lapse with time. It is obligatory to rectify the matter and pay it immediately.