If you have a grease-injector tool for free wheels (I have a Phil Wood FW grease injector) and to lube a freewheel with fresh grease takes a couple minutes. You just need to remove the FW, attach the tool with a tube of grease, squeeze a bit for maybe 15 seconds until old oil and black sludge ooze out the front, then remove, wipe off excess, spin the freewheel to loosen up the grease and wipe off more excess and re-install.

For a more complete overhaul, if there isn't any play to adjust, I don't bother taking apart the FW, and simply soak the whole unit in a solvent tank and spin the center a little to let the solvent work in. Commercial tanks w/ ultra-sonics vibrate and do an awesome job on these parts and you can come back after just 2 minutes and see a ring of grit under the FW if the tank has relatively clear solvent. You can also just let soak in a standard big plastic tub, like for butter, and some degreaser and put your FW in there, and get some nitrile or rubber gloves and spin the center of the FW while soaking in the solvent. Leaving it in the solvent for 15 minutes or so, then taking a hose and rinsing it under pressure, and then soaking in clear water and spinning gears for a minute will help clean the FW.

In a shop, they have compressed air and can use that to blow into the crevices to flush out remaining solvent. You can do that at home too if you have the equipment. Otherwise, I just simply let the part sit in the sun for an hour on a hot day and it will literally get REALLY hot and cook off a lot of water or solvent from inside. Then I repeat with the freewheel injector. The FW will ooze for a week or two and so wiping down the rear drive-train is a good practice. Then the FW seems to just work great for almost a year before I feel the urge to do that again.