Deposit addresses on your account do not expire unless you create more than five of them for the same cryptocurrency. If/when you generate more than this, the oldest deposit address will automatically be set to expire in seven days. You cannot currently choose which deposit addresses will expire.
Note: Only one address for ERC-20 cryptocurrencies can be generated.

If an address is set to expire, it will be clearly marked as such.

We recommend that you stop using deposit addresses that are set to expire as soon as possible. Kraken is not responsible for funds sent to an expired deposit address.

An address marked as "Used Address" is OK to deposit to.