Starizona have a quick guide on optimum exposure for any CCD system.
Here
http://starizona.com/acb/ccd/advtheoryexp.aspx
They have an ideal exposure calculator here
http://starizona.com/acb/ccd/calc_ideal.aspx
All you need to input is your camera and sky background for a given exposure.
It is also worthwhile if you want to get into the mathematics to have a read of the info on Stan Moore's and John Smith's sites.
Stan Moore here
http://www.stanmooreastro.com/
John Smith here
http://www.hiddenloft.com/notes.htm
and here
http://www.hiddenloft.com/notes/acq.htm
The basic take home message is to take 2N+1 exposures that are half or a third of the exposure needed for sky background to overwhelm read noise.
With 3nm narrow band this of course allows for very long exposures. Then the inherent camera thermal and random noise become a problem. That is why a faster optic is preferable.
Bert