Cenvat credit on sales return

We are doing manufature and sale of own branded garments and paying central excixe duty from fy 01.04.11. Since we are selling to retailers, there is regular sales retutrn from the parties. Kindly confirm whether we can take the cenvat credit on sales return made by retails. If yes what is the procedure and time limit for sales return

Thanks

Arul Murugan.M