Part 91 does not require any takeoff mins under IFR. Legally you can take off in zero/zero. There is a line between being legal and common sense. As a good rule of thumb, if the ceiling and visibility are lower than the minimums published in an instrument approach back into your departure airport then it's probably not a good idea to take off right away. 91.175(f)

Part 135 says that you cannot takeoff under IFR from an airport where weather conditions are at or above takeoff minimums but are below authorized IFR landing minimums unless there is an alternate airport within 1 hour's flying time of the departure airport. 135.217