You can use them as often as you wish. I use mine about every 4th or 5th cleaning. Here's a good article: http://www.surplusrifle.com/reviews/copperout/index.asp
and here's s post from another board I frequent:
SO! As promised many moons ago.... The Electric Bore Cleaner.
I tried to reduce the drawing I had made up into something that could be posted but it got too tiny to view so I'm going to assume that anybody reading this will have a general idea of a basic electric circuit with a line running out from the positive end of a battery and returning to the negative end. That is all this device really consists of. The power source is a single, "D" cell flashlight battery. I like the rechargable ones for obvious reasons. In this circuit, a wire is run from the positive lead of the battery, through a simple ON/OFF switch, and out to a 3Ft length of wire with an alligator clip attached. From the negative pole, a wire is run to another 3 Ft length of wire with yet another alligator clip attached. This time, between the negative pole and the clip, you're going to wire in a 0-100 Milliamp DC panel meter. I like to assemble all the components in a Radio Shack 'electronic project box'. They come in all sizes and make for a neat job. Radio Shack also sells the battery holders and 3 ft test leads with alligator clips already attached. With the exception of the Panel Meter, you can probably pick up almost everything you need for the job at Radio Shack.
The next item you'll need is a 1/8 or 3/16" diameter piece of spring stock, about 3 ft in length. Usually hobby shops have this. Any kind of steel rod will do provided it doesn't have alot of flex in it. On one end of this rod, you're going to shrink into place a length of the heat-shrink wire wrap cover -again available from Radio Shack. Harbor Freight has them as well. The ideal here is to cover about 3" of the end of the rod, including the butt end. No rod exposed on that end. Over the other end of the rod you're going to slip another length of the heat-shrink but you're not going to shrink into place. This piece of tubing will be slid up and down the rod as needed for different barrel lengths. The shrink wraps, both fixed and sliding, will keep the steel rod from grounding to the barrel. It's this rod that will recieve all the copper from your barrel so polish it up well, removing any scale and oil before you get to working on it. You're almost done.
Lastly, you need some Parson's Brand Non Sudsing household ammonia. It's blue in color. Why Parsons? Dunno! The guy who designed this unit said to use it, and it only. They sell it nation-wide. Maybe it's not important. Just don't use "sudsing" ammonia.
To Use the Unit:
Clean the powder residue from your barrel. With the bolt removed, insert an appropriately sized cork into the chamber. Prop the gun with the muzzle up. Using a 10CC syringe or a small funnel pour the ammonia into the bore, leaving enough space to insert the electrode (steel rod) without overflowing. Ammonia will dull a glossy finish on a gunstock poste haste so, if it's deluxe wood, remove the barreled action. Otherwise, tie an old T-shirt around the barrel below the muzzle to catch overflow. The Ammonia will not harm the gun's blueing. No guarantees on nickle, though.
With the rod in the barrel (shrink wrapped end towards the chamber) , and the ammonia filled up to the top attach the positive lead of the unit to any metal part that is in circuit with the barrel: the trigger guard, rear sight, reciever ring...what ever. The negative end will attach to the exposed metal end of the rod. The sliding tube will lay between the rod and the barrel preventing the rod from shorting out to the barrel proper.
Flip the switch. The meter should have a brief (1-5 second) spike, then start drifting downward. It will settle out at some point. If the meter stays spiked, check to see that the exposed metal of the rod isn't touching the barrel. Don't freak out. There isn't an arc of electricity carving into your barrel. Just reposition the rod until the needle starts downward. After a few minutes you'll see the meter needle drop quite a bit. Turn off the unit, remove the rod, and wipe off the balck gunk that has accumulated on the rod. It's powder residue and other junk carried over by the current. When you wipe the rod you may already see traces of copper. Reinsert the rod, top off the ammonia, and start it up again. The meter will climb up past it's previous low point. Now you can leave it alone unless you're cleaning an ancient Lee Enfield or such with 80 years of copper fouling! About 20 minutes should see a normal rifle cleaned. There will always be a few milli-amps of current passing through the solution so when the meter reads below 10MA I usually figure it's done. When done, remove the rod, pour out the ammonia. (It will be a crazy blue color) Remove the cork. Run a patch of H20 through the bore, then patch it dry. A light patch with a drop of oil follows. Done.
Safety: The fellow who designed this rig put it into print in a shooting journal some 20 years ago. The next issue, the Editor (an Ex RCBS employee) stated that he'd "heard" that this unit would etch barrels and the electricity could arc inside the barrel, and that possibly, the very iron could be sucked from the steel itself. The designer posted a retraction of the article, claiming that RCBS had threatened him with a lawsuit over what they deemed "patent infringments" on their electric bore cleaner. Well, I smelled a rat. I was in college at the time and our school had a very good chemistry lab. Long story short, with the help of the head of the Chem dept., I ran an experiment on a section of barrel where by I induced current for 120 continous hours. Every day the section of barrel was examined for surface irregularities and weighed to the 10,000 of a gram. No change in weight showed nothing being "stripped" from the steel. There was no change in surface finish. I have used this device for 20 years now on $30 junkers and $5000 collector's guns with no worries. As some of you know, I am a cast bullet junkie. Before this device I would run through quarts of "Sweets 7.62" to remove the copper fouling from each new treasure that crossed my bench. As all cast bullet shooters should know, copper fouling causes inaccuracy and leading. This "de-plating machine" has been a godsend.
By today's standards, this unit costs about $30 to assemble. Built to the specifications I outlined, it should be safe when used as described. If you change anything you're on your own. If you choose to build this unit, you do so at your own risk. Don't you hate having to read that?
Good Luck. ~AMMOe