how quickly it exchanges the air in the room depends on the cfm rating of the fan and the number of outlets that the fan services. I think the code states that a 20 cfm is necessary. what is cfm rating on your fan? number of ducts? do you operate the fan continuously?

I doubt many people measure the actual airflow at the given outlet.

The HRV fan will be at least 20 cfm, most probably 30 cfm. The exhaust fan can be as high I can ask. Basically the default comes with HRV fan and we are thinking of getting exhaust fan instead

International Residential Code

R303.3 Bathrooms.

Bathrooms, water closet compartments and other similar rooms shall be provided with aggregate glazing area in windows of not less than 3 square feet (0.3 m2), one-half of which must be openable.

Exception: The glazed areas shall not be required where artificial light and a local exhaust system are provided. The minimum local exhaust rates shall be determined in accordance with Section M1507. Exhaust air from the space shall be exhausted directly to the outdoors.

A separate exhaust fan pretty much defeats the purpose of an HRV. It is more usual to just have one or the other. Ask about a booster switch for the bath exhaust going into the HRV. Also, make sure that your local code allows an HRV exhaust in the bathroom. If necessary, get a higher CFM HRV, but both is counterintuitive. The HRV should already be running at 20 CFM continuously, anyway, and at a much lower noise level than a 50 CFM bath fan. 20cfm will clear the room in 8.5 minutes, while 50cfm will clear it in 3.4, but both will be slowed down to about the same level when the door is still closed.