The expiry date is mentioned in your Amazon account - usually it is one year from the date you loaded the balance. For any gift cards and vouchers issued by any entity, they have a expiry date because they want the users to spend the money within a reasonable amount of time.
Welcome to our Community and thanks so much for your first post - I am sorry to understand that there has been an issue with a welcome offer. If you log into your online account, you'll be able to locate your contract; is the offer listed there?
If so, then you should have received the voucher via email within 90 days of your installation.
If it is not listed then the voucher would not have been offered by ourselves and may have been a 3rd party. We have seen a few customers where the offer was given via a 3rd party site and not ourselves; if this was the case, you will need to chat with the 3rd party you signed up with and they can chase this for you.
If you need further assistance once you have checked your contract, please do pop back and let us know