Many students ask this question. You should collect enough scans until the signal-to-noise ratio is adequate for your needs. For NMR measurements (where saturation is not a problem) the signal -to-noise ratio increases as the square root of the number of scans. In order to double the signal-to-noise ratio in a particular spectrum one must collect 4 times as many scans as were collected in the original spectrum. Take a look at the figure below where spectra are plotted on an absolute intensity scale against the number of scans collected.
If the signal-to-noise ratio is only 2 after 1 hour, it will be < 7 after 12 hours and only 16 after 64 hours. If you do not see a signal in 1 hour it is probably not worth running the sample overnight!
