how long does the Background check takes from start till finalize

[waiting_4_ottawa]

Greeting everyone,

We did our Medical exam then after 2 weeks we got a call from IOM to do the COA culture orientations class all that done last month in Oct-2012.

I contact the Canadian Immigration office asking when we got the visa and they said your case just like other immigration cases and under background check !!
and it takes several months !!

for me I was expect the do the Background check after the interview or before it not at the final stages..!!

so anyone know how long the background check take? and when the started it before the medical exam or when we do it or after the cic office received the result..
hope you share any experiences with me..

thanks