Cheque expires after 6months from the date of issue. From April'12, it will be 3 months.
If cheque issued on 1-1-2011, it is valid upto 6 months i.e., 30-6-11, after that you cannot present the cheque for encashment. either drawer of the cheque has to alter the cheque with his signature or can issue new cheque, after expiry date.