How long does it take to get the refund of tax?

I m freelince artist on tv. The parties whom I worked with cut the TDS while paying. At the end of the year my income was not taxable. I filed the returns for the year 2006-07 on 11/08/2006 and for 2007-08 on 13/11/2007, But I didn't received any refund yet. I filed the returns thro' proffessional advisors. what could be the reason? who 'll guide me? was there any error from my side? I hv been to the IT office too, but couldn't get satisfactory answer. Pl give answer in detail .