Hi Moh
Shutter speed? Are you trying to use the webcam as a still camera?
You really want to take a series of exposures in the form of an AVI movie - not a single still. Get hold of a copy of Philips Video Lounge software which will capture movies for you. You then open the resultant movie file with your preferred stacking program, such as Registax, to process the stream of frames into a stacked image. The movie length is up to you - they are pretty disk-hungry so maybe start with a 500-frame burst. With a Dob mount, you want to start exposing the planet (Saturn in your case I believe) as is comes into the field of view. Keep watching your monitor and then stop the exposure as the planet moves and approaches the opposite side of your FOV - don't let iany part exit that FOV. Registax can work with this, and will track the planets movement frame by frame as it progresses. I can show you how this all works on Saturday night if you want to bring it to Stargard. Are you coming?
There's plenty on it in here