Not sure what pics you've been looking at but I get about 1/2 to 3/4 inch of oily blowby out of my HEMI powered 300C SRT8 in 3K miles. I change oil in that rig at 6K miles and have never been close to a quart down....ever. 2 inches of material in that can is less than a 1/4 cup of fluid.
Some of the material going to the catch can is water so it's not all oil going in there.
By all means keep an eye out on the oil level as you normally would but don't worry about running out of oil by adding a catch can. Remember, all that oily stuff gets sent from the crankcase into the intake to be burned. Your gonna lose that fluid in any event, with or without a catch can.
