Short answer: it doesn’t expire.
You do not have to use an Unlock Code right away. It can be used any time after you buy it. Once you use the Unlock Code, you will receive your test on your dashboard. That test must be taken within 180 days.
So you can wait as long as you want before taking your test. So, take your test now, or take it later, or take half a test now and then the other half later–the choice is yours!