Monthly credits do not roll over from month to month. On your monthly billing date, your remaining credits will expire and your credits will replenish based on your plan's monthly allotment (i.e. 1,250 credits on the $29/mo plan).
That said, you can also purchase additional credits that will roll over from month to month. These rollover credits will be the last credits used each month and never expire. You can purchase rollover credits in your account by going to Settings > Plan.