OK, it is never a good idea to drain a humidifier into a sewage line of any kind,..but to answer you Q: your vent and or main line should have no problem handling the GPH you described. Any connection to a sewage line has to be trapped as to not allow the smell to come back into the humidifier.
If your humidifier has a resivour to hold the water, your ok. If it's a flow though, your still OK but watch the smell.....!!!!! Give the Model # and I can give you more Info.