It depends greatly on the bulb. Some PR base bulbs with a metal ring, like those used in Maglites, take hardly any longer than the time required to brush the paste on the bulb. A hard glass bi-pin Welch Allyn bulb may take 15 minutes to get the desired result.
I think it's best to frost in stages so that you can test the look of the bulb in your light between applications. It's important to frost as lightly as possible; only as much as is necessary to remove any artifacts and clean up the hotspot. Any further frosting will only serve to reduce the "brilliance" of the beam. An example would be examining the details of the skin on your arm under a bare overhead fluorescent fixture versus one covered with an acrylic diffusing panel; it's the same amount of light, but the panel causes things to look less sharp.
For Maglite type bulbs, try brushing the cream on quickly and with an even thickness. Once the bulb is covered, leave the paste on for 10-15 seconds, then rinse it off. Dry the bulb and take a look at it. If it is still clear in some spots, but hazy in others, you are almost there. I'd advise trying the bulb in the light right then, not only to check for any undesirable artifacts still showing, but to judge how much the "brilliance" has dropped. If you then choose to frost it a little more and check it in the light again, you will have a mental reference of the "brilliance" trade-off that came with each degree of frosting.
The end results are going to be disappointing with some reflectors and pleasing with others. As others mentioned, with etching, there is always going to be a decrease in sharpness, or output, or throw, or whatever one may choose to call it.
.One thing I learned the hard way when etching is to be sure the bulb is 100% dry before you run it-or again the bulb will go 
