Monthly credits do not rollover from month to month. On your monthly billing date, your credits will expire and replenish to the allotment of your plan (for example, 1250 on the $29/mo plan).
That said, you can also purchase additional credits that will rollover from month to month. These rollover credits will be the last credits used each month, and never expire. You can purchase rollover credits within your account by going to Settings > Plan.