First, replace the PCV valve. If you’re really lucky, that might solve the problem.
Generally with significant oil loss, there’s either an external leak or internal consumption. Not all external leaks drip a lot onto the ground - most of it might be evaporating (though usually you’ll smell hot oil in that case) or getting blown away while driving. You can tell for certain if there’s drippage by laying a sheet of newspaper or cardboard under the engine area. With the engine warmed up and the sheet in place, run the enine for 15 minutes. For 5 of those minutes, rev the engine to a speed of 2,000 or so rpm. Then shut the engine off and wait at least 30 minutes. Check the sheet for any drips. Whatever the results, have the car thoroughly inspected for any signs of oil leaks. These checks should pretty well establish whether or not there is any significant leakage.
Internal oil consumption is typically either due to worn piston rings or faulty valve stem seals (but be sure to eliminate the PCV valve first). The rule of thumb is that 1,000 or more miles per quart of oil is within the normal range of oil burning, 500 or less miles per quart is bad enough to warrant repair. Fixing worn rings is essentially overhauling the engine - likely cost 2,000+. Replacing valve stem seals usually costs less the 500. Noticeable blue smoke is common with these problems, but is not always present.
Internal oil leakage is pretty much confined to a head gasket breach. There will be noticeable oil in the coolant, visible at the top of the radiator and/or in the reservoir. Usually it won’t lose enough oil this way to account for the amount of loss described.
It can be helpful to get an accurate miles per quart figure. Get the level to “full,” then check it often enough that you can catch it the “low” mark (the difference represents one quart). This is when you want to top up, so until it’s fixed check it at often enough to continue catching it at the one quart low point.