It's accomplished by exposure to a source of gamma radiation, usually a rod of cobalt 60, and such sources are commonly used to sterilize equipment and supplies used in the fields of research, medicine, pharmaceuticals, cosmetics and even food. Cobalt 60 source gamma exposure doesn't leave items with any residual radiation. There are various companies in the US that offer sterilization services and so mine owners or mineral dealers could utilize these companies as well. The time of exposure would vary on the size of the cobalt 60 source and how active it is...  but for this application it would probably be on the order of hours. I used to work at a facility with a cobalt 60 source and there was a chart showing how long of an exposure you would need to hit a certain desired dose of radiation, and the chart changed every week or month as the cobalt 60 source slowly decayed over time. I think that the smoky color change that occurs in irradiated quartz crystals is not fully reversible. I anticipate that leaving the irradiated crystals in the sun for a prolonged period of time would bleach the color somewhat but never fully. Plenty of other minerals are gamma irradiated by mineral dealers fairly often to improve color, such as spodumene var. kunzite, topaz, fluorite, and tourmaline.   

The smoky color is stable.  I have several irradiated Arkansas quartzes that were treated at least 25 years ago.  They are essentially black.  They have shown no loss of color but they are not exposed to sunlight.  They are actually rather nice crystals.  Always wondered why someone would want to irradiate them in the first place.  I use them for educational purposes to illustrate effects of various mineral/gemstone treatments.  Heat treatment should remove the smoky color by eliminating the color centers caused by the irradiation process.