Currently, unused text messages credits expire at the end of each calendar month.
With the new pricing we announced, one of the benefits is that it'll enable us to develop functionality that allows some text message credits to rollover from month-to-month. This way, communities can accumulate more text message credits than they'd otherwise receive in a single month.
This feature request will be used to track our progress on this; give it an upvote if you're interested in following along!