SMS credits have an expiry of 3 months after the date of purchase.
To renew your expiry date all you need to do is continue to purchase credit (in other words, use your account!).
Every time you purchase credit, your FULL BALANCE expiry date moves to the new date 3 months from that purchase.
On 15 May 2013 you purchase 125 SMS. These SMS will expire on 15 August 2013.
On 27 May you send 25 SMS, so your remaining balance is 100 SMS.
On 10 August 2013, you purchase another 125 SMS. Then your total balance (100 + 125 = 225) will expire on 10 November 2013.
So remember, if you see your balance expiry date is approaching simply top up your account so your full SMS balance is valid for another 3 months!