Typical variations for persons with otherwise normal sight are about 2 magnitudes. That is a large improvement. Brief exposure to bright lights wipe out this improvement almost immediately. Thus viewers should shield their eyes from any light while viewing and especially from very actinic light. Dim red light is the least damaging but even that causes some decrease in acuity.
Adaptation also depends on the size of a spot of light shown on the retina in a complex way.
The best advice is to severely limit exposure of the retina to any light to retain maximum brightness acuity. These numbers and variations from person to person show why some viewers claim to see Mag 8 stars regularly while other have trouble seeing Mag 4 stars under similar conditions. People's brightness acuity simply varies by a great deal and may depend significantly on the use of tobacco, alcohol and other chemicals. On a broad average, most persons can see Mag 6 stars on a clear dark night. The structure of eye is complex, here are outlined only a few factors that directly affect astronomical viewing.