That's right. You have a 6 month window to use the free e-certificate that you earn annually with this card. (The Priority Club credit card also gives you an annual free night, but you have 12 months to use it.)

If you travel (and would want to use the free e-certificate) during the summer months, now is a good time to apply for the Marriott card. By the time you are approved, you will be earning an e-certificate each year sometime in April. That would give you an April - October window to use the e-certificate.

One more tidbit: if you know you will have an e-certificate coming in early April, but let's say you want to reserve a room for May, go ahead and reserve the room in advance (as if you are going to use your Marriott points). Then, when the e-certificate hits your account, apply it to your upcoming stay. If you wait too long (don't reserve a room until the e-certificate arrives), you might not be able to get an award room in your desired location.