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There's a few, such as the astronomik CLS ccd, which I have. They work well at knocking down the sodium and even some skyglow. However there is a strong blue cast to the image as a result. Some of us have yet to work out how to restore the yellow stars, and those who have figured it out choose not to tell...
To be honest, I'm not convinced they help much overall with mono CCD.
Sub duration is a function of the object, lp severity, your sensor, the f-ratio of the scope, etc. the best way is to experiment, and there are some tools in pixinsight that will give you an indication based on analysis of an image.
Cheers,
Andrew.
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