This is not correct. Progress needs to be attached to the aft port of the Service Module in order to reboost the station. Not coincidently, the Progress on mission ISS-89P is docked there now.

Blair Ivey: The air loss is actually not the main concern. The main concern is the fragility of the Zvezda module itself, caused by stress fractures. It is still used for Progress dockings, and Progress is still used to raise the orbit of the station. There is a growing fear that the module itself could crack catastrophically during those operations.

The habitable volume of Zvezda is given as 1650 ft^3. Leakage rate per 24/hr period looks to be around 3 lbs/day. Atmo in ISS is given as Earth ambient at Standard Temperature and Pressure (STP). Air at STP masses 0.07967 lbs/ft^3. Some arithmetic yields about 37.5 ft^3/day reverting to Space, or a little over 2% of the given volume.

I know that the Russian side has a greater volume than the one module, but even if you cut the volume ratio in half, that’s a lot of difficult-to-replace resource. Other variables constant, loss would likely follow a natural log curve as pressure decreased, perhaps reaching an ‘acceptable’ level, if the atmospheric pressure was not reduced below minimums.