I never had any luck trying to burn off the gunk once it had already built up on the coil, in fact I found that I could hold the power on for ages with the side of the atty getting too hot to touch....and yet I would still never be able to see the coil glowing red and worst of all it usually ended up with a dead atty. That had been the main reason I had thought this was a pretty hopeless idea at first.
I would only recommend doing the dry burn if you are going to do it daily from new and the gunk isn't given a chance to build up in the first place, otherwise it's probably far better to stick with alcohol or cola baths or something..... as long as I do it every day it only ever takes a couple of seconds to see it glowing red and I stop burning straight away once I can see it glowing.
I have to admit it is a bit of a leap of faith to do the dry burn 'before' the atties performance is diminishing but by the time I tried this method, I was so fed up with never getting any longer than two weeks out of an atty anyway that I thought 'what the heck!' now my atties are all lasting well over two weeks and best of all with no loss of performance at all.
If you are already getting months out of an atty ....please 'don't' bother trying doing dry burns ..just carry on doing whatever it is you are already doing!!
My latest atty is now on day 11 and still performing just as good as new and even that is better than anything I had before I tried this.