The number of hours you require can be a fairly deceptive measuring tool.
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For a whole face, I've heard between 300-400 hours of electrolysis.
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Regardless of the number of hours, it's typically going to take you 2 years to complete an area, because not all of the hair is growing at the same time.
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If you're only doing a certain area, and not the whole face, you're probably best left judging how many hours to expect...by waiting to see how long it takes them to clear that area every week. That duration will slowly diminish as the hairs are killed, but it's a good point of reference. Then you can assume maybe half that time after 10 months, and trickling down to nothing from there. (Hair growth/resilience is highly variable, but this is one way you could look at it)
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I started electrolysis in July of 2008, doing on average 2 hours per week.
(not enough for the whole face, but you do what you can afford)
From Day One, we started on the upper lip, until it was cleared.
(A note on the word "cleared": It doesn't mean "finished" it just means that you removed all of the hair that was visible at that time. More hair will continue to grow, and you'll need to "clear" the same area the following week.)
Then we continued to clear it every week since. My hair was extremely dense and very coarse when I started. Now, after a year-and-a-half it's very light and sparse, but we still clean what's left on the upper lip every week.
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One bit of advice: If you have large areas to do, don't let your electrologist use the 'buckshot' stategy of a-little-here-and-a-little-there. Since hairs are really only vulnerable while in the anagen phase, it's best to get an area clear every week (so you know that everything in that area is in the anagen phase) (anything treated that's in the catagen or telogen phase is just wasting your money) and then keep enlarging that area.
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To be realistic, look at this as a 2 year commitment until you're through, but take heart in the fact that your face will gradually look better and smoother. Even though it won't be completed at the end of 1 year, you'll notice a great difference, and makeup will be able to cover things more easily.