There's never any shortage of people saying ludicrous things about gum printing, and a few years ago, there was a self-appointed gum expert proclaiming that the correct exposure time for a gum emulsion is six minutes. This of course is arrant nonsense; the correct exposure time varies widely and depends on many different things, as we'll discuss below.
First, how to determine correct exposure time? Use a step tablet (a Stouffer 21-step is not expensive; I keep six of them handy at all times) to determine this. The test strip, on the same paper with the same emulsion and same light source, under the same environmental conditions, that you intend to use for the print, will give you the optimal exposure for that emulsion on that paper under those conditions. If conditions change, or if you mix a different emulsion, then you need to make a fresh test strip. [Test strip here].
Some people recommend exposing til the bottom two steps are the same color and value (to ensure that you've exposed for the maximum DMax available from that particular emulsion) and then reducing time til you get just one step that darkest value. In my own experience, DMax isn't the crucial thing, since DMax is reached before optimal exposure is reached. Yes, DMax is necessary, but not sufficient; I find that the main thing is to expose til you retain, in a normal development time, the maximum number of steps for the emulsion, and if you do that, the DMax will take care of itself. Since To illustrate what I mean by that, consider this set of test strips:
These strips were exposed, all on the same piece of paper, at (from left) 1 minute, 2 minutes, 3 minutes, and 4.24 minutes. The correct exposure, as determined by separate individual test strip, was 2:30 minutes; the strips don't reflect that very well because the development was chosen for the most-exposed strip rather than the optimally-exposed strip. But never mind that, because I chose this set of strips not to demonstrate optimal exposure but to illustrate the point that the amount of exposure required to achieve DMax is less than the amount of exposure required to achieve the maximum number of steps; in other words, DMax is a necessary condition for correct exposure, but not a sufficient condition. You'll note that the background area surrounding the 1 minute-exposed strip at left is as dark and saturated as the background area surrounding the strip exposed for 4:24 minutes at right. And the DMax (value and saturation of bottom step) within the step prints is the same for the 2 minute strip as for the 4:24 minute strip, even though 2 minutes represents a 30 second underexposure. In other words, DMax is normally achieved before the optimal number of steps is achieved, and in my experience it's better to expose for the latter than the former.
Influences on Exposure Time:
Light source: it goes probably without saying that a major determinant of exposure time is the light source being used.
Dichromate concentration: Dichromate concentration is another very important determination of exposure time, as there is a strong and linear relationship between dichromate concentration and speed of the emulsion. This is demonstrated not only in my own tests but is a relationship well established in laboratories for decades.
Humidity: There is a very strong and almost vertical relationship between humidity and speed: the higher the ambient humidity, the faster the emulsion will print.
Pigment concentration: a more heavily pigmented emulsion requires more exposure than a less pigmented emulsion, using the same pigment. Different pigments may require different exposures depending on characteristics of the pigment, although there's no clear identifiable relationship between characteristics of pigments and speed. For example, I don't find any evidence to support the commonly held notion that pigments in particular hue ranges require difference exposures because they act as UV filters depending on their hue range.
Paper: Different papers print at different speeds. If you always print on the same paper, this won't be an issue, but if you change papers, you may find that you need to change your basic exposures.