Is there a time limit within which I can use the 80GBP credit? i.e., if I only run the server part of the time, can I use the credit over the period of say, a y

Is there a time limit within which I can use the 80GBP credit? i.e., if I only run the server part of the time, can I use the credit over the period of say, a year? or does it expire after a certain time has passed?