It astounds me that people don't realise they need to clean their sewing machines out fairly regularly! If you want something to work well then you need to look after it right?
To be honest it is pretty easy to keep your machine running smoothly with a quick clean out every now and then. Here's a quick "How To" to help you on your way. (This is written for top loading machines)
You will need:
A small screwdriver
A lint brush (or cotton buds if you don't have the brush)
Sewing Machine Oil
8) Once you are satisfied there is no more dust to be seen you can pop the bobbin case back in, secure the metal plate back on top with the screwdriver, replace the presser foot shank and needles and sew your heart out again! Lol!
I don't clean my machine regularly enough, I would say it would need a good clean after say 8 hours of solid working on it. But it depends what fabric you are sewing with as some lose more of their fibres along the way and therefore make more of a mess than others.
Hope that helps.