My Newson doctor says the fastest increase is every 4 weeks. That's what she advises me.
I've just ignored her and increased to 6mg of Sandrena after 1 day at 4mg however.

4mg had me awake until 3am with all my worst night time symptoms - which improved as soon as I increased the next night. I'd rather slightly overshoot and then do bloods and find out and reduce down than spend half my life incrementally increasing HRT. That's why I am now 3 years on HRT and still have not got my E over 325pmol

I find that the symptoms I have improve within about 2 days of an increase, definitely by a week I can tell a difference. None of this 3 months business - I think some side effects like breast soreness, bleeding patterns and that kind of thing can take 3 months to settle and for some reason all side effects have been lumped together and we're just told to wait that long for everything. When some of us know our own bodies a bit better than that...
Maria, did you try utrogestan? I'm just wondering if your occasional mood blips could be low progesterone. The Mirena will deal with the uterine lining building up but it's not going to replace your lost progesterone. There is nothing wrong with you taking progesterone as well as being on the Mirena. The NHS sometimes doesn't like to prescribe both because they seem to think the entire role of progesterone is to stop the uterine lining building up so you don't need both - but P does much more systemically than this. However if you have bad P side effects, then ignore me....