Seed banks are mostly self-refreshing. Seed viability decline during storage is measured and modelled for. A sample of seeds is taken out of storage and grown to breed a new batch of seeds after an amount of time based on the rate of decline of that sample.

So a batch that loses 20% viability every 5 years will be regrown to seed after a shorter amount of time than one that loses 2% viability every 5 years.

Source: was a seed germination and dormancy researcher at the Millennium Seed Bank