Is zakat obligatory on money saved for years, when the amount on which a full year has passed is unknown, after a large portion of it has been spent?

Zakat becomes obligatory on money if it reaches the nisab and a full year (hawl) passes over it. It is not waived due to ignorance or by spending the money after it became obligatory. The questioner should exert effort to estimate the time when the money reached its nisab, and then pay 2.5% for each year, along with repenting for the delay in paying Zakat. If she has spent the money, the Zakat remains a debt upon her, which she must pay whenever it becomes easy for her.