I’m wondering how long I can leave between electrolysis treatments on my upper arms? Would 7 weeks be OK, or would I be risking letting the hairs go out of anagen phase by then?
The phase of the hair is not a determining factor for the Electrolysis is effective.
The determining factors are:
Incorrect insertion.
Current intensity insufficient.
Current application time is too short.
Any of these circumstances means that the hair is drawn by force and, therefore, the reappearance of a new hair in the follicle treated incorrectly.
bea25: She is right that the phase of the hair doesn’t have to be a determining factor. That being said, many electrologists don’t work on telogen hair, only anagen. Maybe they don’t trust themselves to properly disable telogen hair, or they find it is not as comfortable for the patient (I think telogen hurts many times more than anagen, but that’s just me).
So, if your electrologist really only trusts herself to get results on anagen hair, then I wouldn’t let the hair get older than maybe 6 weeks in any one area. But the hair takes a week or two to reappear in the first place, so added together, I think 7 weeks after the initial treatment would still be within that window.
That’s great news - thank you MPK! And it’s good to know it’s normal for hair to be reappearing within 2 weeks, as well. I usually have a new crop out by the third week, which can be a little disconcerting…
Nah, pretty normal in the beginning to middle of the treatment. I would be worried if you still saw the same amount of hair crop up every week after a year of treatment though. 
Luckily I can tell I am making some progress, simply because I can see little red dots of hyperpig instead of hair in some areas! The lesser of two evils, as far as I’m concerned 
How long a hair is in anagen depends on the location on your body. Facial hair has the shortest hair cycles and needs to be treated at least once every 3-4 weeks or so if possible. Upper arms can wait a week or two longer.
Six to Eight weeks is a good outside limit if one can get full clearances each time. That’s why working every 2 to 4 weeks works. If one manages to get the full clearance during that 8 week time frame, one is still in the box. Around 12 weeks, one has surely missed some hairs.