Depends on how full you want them to be.
Two and a half times the width of the window is a rough guide.
HTH,
emerald
http://www.fabricandhomebonanza.com/how_to/draperies.htm
http://www.mtdrapes.com/measurepage.htm
http://www.taunton.com/threads/pages/t00082.asp
This site has a calculator
http://www.designersketchbook.com/designersketchbook/draperycalc/
Dora
Thanks all.