If you have more than 6 months in USA (present or past, yes you need a FBI certificate) and it is also recommend to have one from the state(s) where you lived.
To get an FBI certificate, I would say around 6 weeks if you specify (on the application and on the envelope that it's for Immigration in Canada.
I'm on my third application (due to late QSW) and 6 weeks to 8 weeks are the different timing I got.
Good luck!