It depends on which Gateway you will be using.
For the Low volume gateways: Enterprise / Online Gateway there is a minimum top up of 15 EUR.
See website: www.cm.com
For the High volume gateways: Global Gateway (priority) there is a minimum top up of 250EUR.
See website: [smsgateway] (https://www.cm.com/products/text/sms/sms-gateway/)
Your credits are valid for one year.