Julian,
Are you using the grating as a objective grating?
Janet S did a survey of Cygnus with her objective grating and Canon 350D, 85mm lens and tracked exposures of >30s to record the spectra in sufficient detail for "survey" - obviously if any "suspects" are found then you could have a more detailed look.
(Grating Spectroscopes-How to use them, p27)
Probably be a bit quicker - say 5 years or so
