Ryan is right with these machines you do get through the cha a lot quicker that if you hand sharpen. If you don't let the chain get to blunt, you can just give it a quick tickle to help keep the edge on it.
Dario if you are having trouble with the raker height you can get a depth gauge to help get it right. I don't know what saw/chain size you use but here is a link to Stihl.
http://www.stihlusa.com/chainsaws/acc_filing.html
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All you do is place it on top of the chain and use a flat file to take the raker down to the gauge.