All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
1. As a general rule, depositing money in a usury-taking bank is not permissible. If you do not have any Islamic bank and you fear that your money will be destroyed or stolen, only in this case, you may deposit your money in usury-taking banks. If those banks provide you with interest on your money, then you have no right to possess that interest; so, you should spend it on Muslims' public welfare.
2. If one possesses an amount that reaches the Nisaab (minimum amount of property liable for Zakaah that is 85 gram of pure gold or its equivalent) and, after having this amount, he also receives money from other sources, for instance, he saves from his monthly salary, or receives presentations, gifts or rents on real estates, etc., then he can follow two ways to discharge his Zakaah.
First, if one does not want to spend more than his obligatory duty, then he might maintain a calculation of the growing money and pay the Zakaah of every amount on its time; i.e., after one lunar year has passed after possessing it.
Second, if a person is used to paying Zakaah on a certain month (e.g. each Ramadan), then it is better that he pays Zakaah on the whole amount regardless of when it exceeded the Nisaab. This latter practice is preferable if the person wants to be generous to needy persons, he is then prepared to spend the additional money to benefit Muslims, and he receives more reward for doing so. Know that paying Zakaah before completing a year is permissible; it is considered as early paid Zakaah. Therefore, you may guide people to the latter way of paying Zakaah by explaining them its merits.
For more benefit on Zakaah, please refer to Fataawa 165375, 401332, and 379973.
Allah knows best.