Monthly credits do not roll over from month to month. On your monthly billing date, your credits will expire and replenish to the allotment of your plan (for example, 1,250 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 within your account by going to Settings > Plan.