dmilligan: It's all down to pretty simple math (probabilty theory), no need to do too much experimenting. For exposures up to ~1min pixel noise is more or less constant, and it's well known that averaging measurements contaminated with gaussian noise improves signal to noise ratio as a square root of the number of measurements, so when you fix your total exposure (sum of individual stacks), the SNR will scale inversely proportionally to the number of shots. Say, 4x 60s will have two times better SNR than 16x 15s, with the same total exposure (4 min). But at exposures >1min thermal noise becomes important, and thermal noise scales with exposure time, so increasing exposure to >> 1min will no longer improve SNR, as both the signal and noise will grow at the same rate. (Actually, things are likely more complicated than that, this is just a simplistic outline.)