The patient leaflet gives the option of 12 or 14 days.
It's your choice. A lot depends on how well you tolerate progesterone and how much estrogen you use (higher doses may warrant 14 days because more days = less bleeding- usually.)
No one ( or rarely) has a 28 day cycle, especially in peri menopause.
You are better off trying to re-set a new HRT cycle on a monthly (30-31 day basis.)
If you start Utrogestan mid-cycle, you may carry on having a regular bleed each month.
BUT your period may also come early because your own estrogen levels PLUS HRT estrogen, aren't being controlled by that amount of Utrogestan.
It's not possible to keep taking Utrogestan mid-cycle when your periods are irregular.
The way to keep it very simple (my consultant tells women this) is to start Utrogestan on the same day every month. can be the 1st or any other day you choose. If your periods are regular at the moment, choose a mid-point, but if they aren't choose any day.
If your period comes early, keep taking the rest of the dose, and then start again on the same day next month.
Eventually, it will become an established cycle, using HRT.
There are many variations on this which GPs aren't necessarily aware of.
Until recently, the established dose was always 12 days. More recently, it was (I think) Dr Newson who produced a guidance for GPs and said make it 14 days as women find it 'easier' to remember 2 weeks on, 2 weeks off.
This doesn't account however for side effects that some women have and of course we are all able to count to 12!