dlswnfrd - I must disagree with your statements that "time is the controlling element" and "..evacuate the system as long as you have time. You can never over do it." with regards to A/C system evacuation.
You cannot equate evacuation duration with vacuum pump efficiency: a pump with a factory micron rating of 1000 microns will not scavenge a system any lower than that, regardless of how long it is allowed to run. Nor can you compensate for a pump operating at reduced efficiency by allowing it to run overnight. With the inaccuracy inherent in compound gauges, a problem could exist with a vacuum pump and one would never know it. A vacuum of 10,000 microns is indistinguishable from a vacuum of 1,000 microns on a compound gauge, yet the former is an unacceptable level of evacuation for a refrigerant system.
For optimal system evacuation, a vacuum pump's performance should be checked prior to starting an evacuation; most currently manufactured pumps (non-laboratory) have micron ratings of 20 to 50. If the pump is unable to reach the factory rating, then pump maintenance is in order.
Additionally, one should have knowledge of the pump's capacity and minimum pump down times. Robinair's 6 CFM vacuum pump, with a micron rating of 20 (Model 15600), requires 60 minutes to reach a vacuum of 600 microns under laboratory conditions. This two stage 6 CFM pump is no slouch, listed as applicable for A/C systems up to 50 tons, including tractor/trailers, buses and rooftop A/C systems. Smaller pumps, such as 1.2 CFM or 4 CFM, must be expected to take longer to reach the same level.
Given this, a minimum evacuation time of 30 minutes for an opened system is possibly sufficient but more likely not. But how would one know using a compound gauge alone? I suspect Steve is correct in stating that a majority of all evacuations are insufficient.
Clean oil is imperative for peak vacuum pump performance. Moisture and other contaminants quickly deteriorate oil purity, thinning the oil and reducing the pump's ability to achieve deep vacuum. Contaminated oil reduces a vacuum pump's ability to remove moisture yet how many do-it-yourselfers routinely change their vacuum pump oil, let alone even know that it is part of vacuum pump maintenance? All vacuum pump manufacturers call for frequent oil changes; Robinair, e.g., calls for oil renewal following:
a) evacuation of a system that was overly moisture-laden.
b) evacuation of a system with compressor burnout.
c) oil contamination (oil looks cloudy or milky)
d) failure of the vacuum pump to pull to factory specifications when blanked off to an electronic thermistor vacuum gauge.
e) every 10 hours of operation.
Modern automotive A/C systems are built tighter and charges are more critical. These systems have greater sensitivity to contaminants, including moisture, which makes proper evacuation more critical than ever before.
It is well known that the presence of moisture in a refrigeration systems can cause freeze-up. However, this is not the only problem caused by moisture. Refrigerant oil rapidly absorbs and retains water (which is why you should only use refrigerant oil from sealed containers). Water, in the presence of refrigerants containing chlorine, will form hydrochloric acid, which results in corrosion. The water-formed acid also combines with the refrigerant, resulting in a closely linked mixture of fine globules. This "sludging" greatly reduces the lubricating properties of refrigerant oil, which can be the death knell for certain compressors, such as the Nippondenso 10Pxx series.