This depends on the namespace they are in. The namespace is the toplevel of a BNS name. For example bns.btc is in the .btc namespace. These expire after about 5 years because this is measured in Stacks blocks the actual expiration can vary slightly. After every renewal they are valid for 262,800 Stacks blocks from that moment of renewal.
Some more examples names in the:
.id namespace expires after approximately 1 year (52,560 Stacks blocks).