Ionex is not so sensitive to protein concentration as e.g. affinity chromatography. Im not aware of any low limit for Resource Q column (but it's always good idea to have moderate concentration).
The dynamic binding capacity of the Resource Q column is 40-54 mg BSA/ml as per the data sheet of GE health care and one can load 25 mg protein/ml to this strong anion exchange column. if you have 6 ml of the said column you can go for 150-200 mg protein loading to get good resolution...