The acceptance criteria for a vacuum chamber leak rate test will vary with the application of the vacuum chamber or the operations to be performed in the chamber and/or the materials being placed in the chamber.  In the heat treating industry the purpose of vacuum is to reduce the impact of oxygen on the materials to be heat treated.  Typical acceptable leak rate of a new clean, dry cold and out-gassed furnace with mechanical pumps would be less than 10 microns per hour.  For a diffusion pumped furnace it would be lower (maybe 5 microns per hour) as the ultimate vacuum capability will be expected to be lower. Pumping becomes more difficult as the pressure is reduced, thus one needs lower leak rates.