Well, according to a number of sources, as a rule of thumb you should sharpen the chain on a chainsaw every time you refuel it. You can also tell when it requires sharpening because of the sawdust – a sharp chainsaw produces long strips of wood as sawdust rather than very fine dust.
I found this video on YouTube that was really helpful: http://www.youtube.com/watch?v=qvcCh2XqEPc This is one of a series of Stihl chainsaw videos and they are well worth watching.
You can find the sharpening kit here on Amazon: http://www.amazon.co.uk/Stihl-Sharpening-5605-007-1027/dp/B002YQ4SG0/ref=sr_1_9?ie=UTF8&qid=1355176663&sr=8-9
I found the process very easy to do and it only took around 20 minutes. I reckon that I will be able to do this even quicker with a bit of practice. It has made a massive difference to the cutting performance of the chainsaw. Do make sure that you get the right size sharpening kit for your chainsaw – they come in different sizes to match the chain. Much cheaper than buying a new chain.