With every top up, the amount / credit is valid for 1 year. With every following top up, the time is renewed again for 1 year.
1. You place a first order on 01/01/2016. This credit will be valid until 31/12/2016.
2. Three months later (on 01/04/2016), you decide to buy a new SMS pack although your credit is not over yet. This new credit and the remaining credit of the first order will be valid till 31/03/2017.